xjhm said:
anyone know the mileage diff. between changes for conventional vs. synthetic?
If you read the fine print, almost any normal use of an XJ fits the factory criteria for "severe duty," so the recommended oil change interval is 3,000 or 3,500 miles, not the 7,500 miles they talk about for light duty. I always changed oil and filter at 2,500, partially because I felt it was cheap insurance and partially because it's easier to remember multiples of 5,000. I never burned off enough in 2,500 miles to add a quart, so it probably evened out in the long term
My brother is a service manager for a BMW dealer. BMW's recommended service interval using synthetic is 15,000 miles between oil changes. I kind of figured if BMW can warrantee their engines with a 15,000 mile interval, I could certainly switch to synth and run a 5,000 mile interval. That's what I run. I switch my '88 XJ and '88 MJ to Castrol 5W50 and I change the oil and filter at 5,000 miles. I'm sure I could go out to 7,500, but so far I can go 5,000 w/o adding, and if I try to extend it to 7,500 I'll have to add a quart at $5 per quart, which will partially offset the saving from the longer interval.
The factory has never issued a recommended interval for synth as far as I know, so you're pretty much on your own.