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Oil change frequency for fully synthetic and light use?


NAXJA Forum User
Vancouver Island
I've been having a tough time finding a more definitive answer to my specific case for oil change frequency.

I have a 1999 Jeep Cherokee with 110K miles on it. I live on Vancouver Island, so the climate is very mild year round. I use 10W30 Valvoline fully synthetic motor oil. The Jeep sees about 3000 miles per year in street driving. The average trip length is about 3 miles. How often do I need to change the motor oil? Perhaps every 2 years, or are there emulsifiers in the oil which would have sufficiently broken down by then?
Work with a used oil analysis firm to extend your OCIs scientifically. Otherwise I think the rule of thumb for synthetic is double that of ordinary oil.
Work with a used oil analysis firm to extend your OCIs scientifically. Otherwise I think the rule of thumb for synthetic is double that of ordinary oil.

Under usual conditions. I would be hesitant to leave oil in for years at a time.. But as far as type and viscosity, man, that's a can of worms! Everyone has their own opinion and some people can get heated about it. Personally I use 15w-40 rotella and change at 3k. Which is about once a month for me.
I've been running Amsoil for 50yrs but I still use the regular service change schedules just because I care about my motors. If nothing else change the filter and add a quart if just need some time.
I've been using synthetics since the late 80's. Not exclusively though. With out a lab results, I would not go beyond 8K on an OCI. I have exceeded that though to near 12K. It would also be depended on how clean the engine runs. Now, not exceeding that mileage, I've always tried to change synthetics in a years time. It used to be 3 months for dino. The dinos have gotten near synthetic performance in the past 5 years. So, I might extend them to 6 months.
I am running Rotella T6 in my XJ the 5W-40.
I think AMSOIL has a 25K or more OCI on their oils if you used their extended filter. I'm by no means an expert on this. Still, If I were you, 8K and 1 year is what I'd go with.
Work with a used oil analysis firm to extend your OCIs scientifically. Otherwise I think the rule of thumb for synthetic is double that of ordinary oil.

In addition to the above comment, I would add 3 miles per trip put you in a very negative position. The XJ need, at least my XJ need a lot more than that to come up to the proper temperature. The oil and engine for that matter is not hot enough to evaporate all the moisture in it. More frequent oil changes and/or oil analysis may be needed.
I live in Florida. I use Amsoil. The manual allows up to 7,500 miles depending on conditions between oil changes. I change the oil between 12 and 15,000 miles. With 350,000+ miles on my XJ. It have 60 PSI pressure cold and 40+ at idle after a long run.
If I were you, 8K and 1 year is what I'd go with.
Do you mean "8K or 1-yr, which ever occurs first"? If so, I'm at the 1-yr mark. To date, I have 2125 miles on my latest oil change, which occurred exactly one year ago today.

The Jeep sees a 3-mile, 12-minute drive to my kids primary school at 8:30 AM, then a 3-mile, 12-minute drive back home. Then in the afternoon, another 3-mile, 12-minute drive to pickup the kids, and a 3-mile, 12-minute drive home. Temperatures where I live are nominally around 6 - 10 C in the winter, and 14 - 24 degrees C in the summer. Is there not some online calculator or other rules of thumb to narrow in more on more optimised oil change frequency based on user input? At 2125 miles in 1 year, it seems a bit wasteful to be changing the oil again.

I contacted an oil analysis company in Edmonton Alberta, which is the closest I could find to me in BC. The pre-paid cost for the analysis is $62 CAD, which is about the cost for me to change the oil. If I didn't need shipping, it would be $39, but I was unable to find a local outfit in my town.

In the past, I had been changing it every year based on about the same driving frequency, but was starting to think that maybe it could be pushed to every 2 years.
I was told or read to change synthetics once a year. That has been several years ago, say 20. As mentioned before oil is hydroscopic, ie collects water. Synthetics are less hydroscopic that conventional oils. It takes about 30 minutes at temp to 'burn' off the water. This also applies to brakes and other stuff. I would plan to run at least 30 minutes once a week or at least monthly.
The oil analysis should be viewed as to how your engine is doing. If you want or plan to exceed the 1 year or 8K mileage. So, it might be savings in the long run. No, I haven't had one done yet. Also, it might warn you of potential problems.
Also, the 7,500 or 8,000 OCI in manual are for non-severe driving. That is mainly highway driving in non dusty conditions. City driving is or could be considered severe.
Your short drives are considered hard use, thus you should change the oil frequently. contamination build up is the major issue when you only drive it 3 miles at a time. The engine is running rich when cold, water is introduced as a by product of combustion, yet the engine remains cold, so the water and fuel in the oil does not get heated away.

Id switch to cheaper oil so as to afford the more frequent oil changes your type of driving requires. No matter how good and pricey sythetic oil is, if it is contaminated from hard use, like many short trips can do, well you NEED an oil change. As a rule of thumb, one year in the crank case regardless of total miles, if oil shows signs of foaming, or other contamination, then more frequemtly.

The thing to keep in mind is your usage is considered hard use, frequent short trips, without full warm up.

So once a year minimum, no matter the oil type, that is how often I would change it if my rig.
Black1990jeep, thanks for the additional insight. Would it not be more appropriate to consider that 3 mile trip to school as a 6 mile trip? The vehicle is only shut off for two minutes to walk the kids to the door, then it is turned right back on to drive home. Hence, is a 6-mile, 24 minute run time twice a day still insufficient to increase oil change intervals? The engine is up to temperature (210 F) after about 6 minutes of driving.
Sure, you can probably figure it that way, but either way the majority of your uses are short trips, so I'd either change the oil more frequently with cheaper oil, or spend the small amount of money to determine what's really happening in the engine to see if the synthetic is benefiting.

Point is you're spending money either way. The only way to know if you're saving money or oil changes is to have it looked at. If you don't want to do that use ordinary oil and change it as recommended by the owners manual.
In theory, full synthetic oil takes a very long time to break down. That said, I would at the very least, once a year change the filter and top off the oil. And do a full change every other year.

Definitely have your oil analysed. Send in the new oil sample, and used oil sample to get a base line.
I usually change the oil in my fleet of junk every 3000-4000 miles. My trail rigs are changed once a year at the minimum. Sometimes the rigs don't see but a few hundred miles in a year, but once a year at the very least seems reasonable to me. Since I'm obsessed with vehicles, and we have 4 drivers in our household, I change a lot of oil.

Is there an issue with changing the oil once a year for you? It seems like a minor inconvenience and a minimal cost to do for peace of mind.
Another aspect is the filter. For the OP's mileage a standard filter would work. If you're going over 5K OCI, then the extended OCI filters should be used, or as suggested previously, to change the filter and replace oil lost. I use NAPA Platinum filters.
Thanks everyone for their responses. I have decided that I will be ordering the oil analysis. The guy said that they have a database of existing oils, so they don't need a baseline virgin sent in.

I'm interested in the suggestions to use the cheapest filters and cheaper non-synthetic oil and just change it once per year. I went for fully synthetic and the mid-range filters thinking I could extend my service intervals, hopefully out to 2 years. Thus, if I went with petrolium based or a hybrid oil, could I change the oil just once per year with the given driving conditions? My 1979 diesel Benz only sees about 2,000 miles every 2 years and I've been sneaking by with an oil change every 1-2 years using Shell Retega 15W-40. The car burns oil though, so it tends to get topped up about a litre per year. Might get that oil analysed as well...

When I recall my time living in the US, I noticed that those quick lube places for $15 oil changes are on every corner. In Canada, there aren't nearly as many of them and the last time I went in there to change the Jeep's oil [in Canada] the cost was around $70 (and I had a coupon!). So I've been doing all my own oil changes. The next problem came a few years ago when Canadian Tire and other local garages stopped accepting used motor oil. Now I need to drive the oil all the way to the landfill for recycling. I also do not enjoy changing the oil - no matter how much I prepare, it always makes a big mess. In addition, the costs of the filters have gone up quite a bit in the last few years, as has the price of oil (surprising given the drop in crude price). For all these reasons, I'm trying to extend the oil change intervals.
Here are the results of the oil analysis. Unfortunately, I cannot attach images, so I will have to summarise.

Mercedes 240D 1979
OM616 2.4L, 4-stroke, diesel
Shell Rotella T SAE 15W40, 5.7 litres
Oil filter: BOSCH 72126WS premium filter

Last oil change date: 17 Feb. 2018
Last oil change mileage: 176,342 miles

Motor burns oil, thus:
Added oil date: 30 Sept. 2020
Added oil mileage: 177,899 miles
Added oil quantity: 2 litres

Mileage at oil analysis test: 178,202 miles
Date of sample: 2 Mar. 2021
Mileage on vehicle since last full oil change: 1860 miles and 2 years
Mileage on the 2 litres of oil added between changes: 303 miles and 5 months

Usage: Like the Jeep, the Benz only sees short trips of about 2-6 miles in slow moving, frequently stop, city traffic.

Results: All wear metals, contaminant metals, multi-source metals, additive metals, fluid properties, other contaminants, and particles within normal range.

TECHNICIAN NOTES: Data indicates no abnormal findings. Resample at normal interval.

Sorry, it will take too long to type out all the actual values, but nothing was flagged. On the severity scale of 0 to 4, the results was a 0.

Jeep Cherokee Sport 1999 4WD
Inline-6 4.0L, unleaded gasoline
Valvoline Fully Synthetic SAE 10W30, 5.7 litres
Oil filter: FRAM Tough Guard TG16

Last oil change date: 23 Feb. 2020
Last oil change mileage: 107,421 miles

Mileage at oil analysis test: 109,617 miles
Date of sample: 2 Mar. 2021
Mileage on vehicle since last oil change: 2196 miles and 1 year

Note 1: Changed valve cover gasket 1 week before oil sample taken

Note 2: 14 Oct. 2020 @ 108,175 miles, added 8 oz of Blue Devil Rear Main Seal Leak Stop (contains diethylene glycol monoethyl ether according to the first aid info on the bottle)

Note 3: I may have used Lucas Engine Oil Leak Stop a week before the Blue Devil, but I did not record it on my service log. My memory is that I did, and waited a week to see if it was going to stop the leak, it didn't, so I added the Blue Devil (which worked well until recently).

Usage: 8:30 AM, 3 mile trip, car off 2 min, another 3 mile trip, car off, at 2:30 PM another 3 mile trip, Car off 2 min, then another 3 mile trip. That is its core function, plus maybe a Saturday trip to the grocery store.

Results: Silicon levels slightly elevated at 28 ppm. Also slightly out of range were the Sodium at 109 ppm, Molybdenum at 38 ppm, Boron at 76 ppm, Magnesium at 415 ppm, and Calcium at 1110 ppm [perhaps caused by the oil leak stop additives?]. Base No. D4739 was low at 3.13 mg KOH/g.

The overall severity based on the results were a 1 out of 4, with 0-1 being "Normal", 2-3 being "ABNORMAL", and 4 being "CRITICAL".

Silicon is at a MINOR LEVEL; SILICON sources can be abrasives (dirt, Alumina Silica), seals and gasket material, lube additive or lube supplement, and/or environmental contaminant; Base Number is SLIGHTLY LOW. As Base Number depletes, the ability to neutralize acids is diminished. Flagged additive levels are different than what should be present for the identified lubricant. This may have been topped off with a different lubricant, the fluid may be misidentified, or a different lubricant or formulation may have been in use prior to a recent change.

Comments from the lab were that it looks like additives were added to the oil but not noted by the customer. lol, yup! Forgot about the Blue Devil Leak Stop being added in there some time back. Nonetheless, the oil in its present form still appears in the normal condition.

In conversation with the oil test guy, he didn't think the Benz needed an oil change, but cautioned that the oil filter elements break down over time and I may want to change it in the future if I don't change the oil.

For the Jeep, he felt the result was expected for having changed the valve cover gasket and added oil stop products. Changing the valve cover gasket will leach Silicon into the oil and this is the likely reason why the Silicon level was slightly elevated. He did mention that I may want to consider using the thicker 15W40 oil in the Jeep to help with the oil leak. He also commented that there is a large range in what it takes to be called "synthetic" oil, noting that even highly refined mineral oil can be called synthetic. He said that pound-for-pound, the heavy duty diesel is normally a better formula out of the bottle. He commented that diesel oils tend to be more robust than passenger car oils in terms of detergents and anti-wear additives. Diesel oils likely start out with a higher base number and are often formulated to maintain that base number longer than gasoline oils.

I was going to change the oil pan gasket this weekend, which will require an oil change, and wasn't sure if I was going to use the 10W30 synthetic or 15W40 diesel oil. I was a bit concerned that the 15W40 might not last the 2 years I'm hoping to push oil changes, but from the Benz results, it doesn't seem bad at all From the results, it seems like the 10W30 fully synthetic might be OK for an oil change every 2 years with my driving. The Benz was OK at 2.5 years with Rotella 15W40, but of course the car burns oil and I have add about 1 litres per year. The Jeep doesn't burn oil.

The end goal I am hoping for is an oil change every 2 years on both vehicles.

The Benz seems OK for this. For the Jeep, I have exactly 6 quarts of unused fully synthetic, so will use this for when I change the oil pan gasket. If the leak continues, I'll add the Blue Devil again. If leak continues, I'll change the oil after 1 year and switch to diesel oil. The leak isn't all that bad right now, perhaps 2 dime-sized drops per week on the garage floor. If after 1 year no leak, I may push for a 2nd then have it tested again. If I switch to 15W40 diesel oil, I'll have it tested after 1 or 2 years and report back.

Maybe some of you can share some of your test results for comparison?
One other point I forgot to mention that the oil test guy said was that K&N air filters are notoriously bad at filtering very fine particulate compared to a stock air filter. This could be another reason for the slightly elevated silicon content in the oil. When I look into the throttle body, there is a layer of soot on the walls.

One question I still have is how long can a BOSCH premium oil filter last before it starts to break down? Perhaps I can go 3 or 4 years on the Benz...
I guess you have a K&N air filter? There are a number of independent tests showing that K&N filters provide more coarse filtration than most other filters.

As others said, I would expect elevated moisture levels if you're only driving you XJ 3 miles at a time. Make sure the PCV system is fully functional (all connections vacuum tight including at the valve cover) because this system is meant to actively remove moisture. Consider warming up the vehicle for a few minutes before driving, and if possible, drive a little further.