The manufacturers make sure to specifiy how a part is built and what components are used, screw up and they don't buy from you anymore. Things that are a PIA to replace like slave cylinders and clutch parts are a no brainer, the dealers/DC don't want to have to replace them under warranty any more than you want to have them replaced. I also think that this applies to the coolant, I would not be suprised if they have a 'special blend' of coolant, with extended warranties they don't want to have to replace plugged rads, leaking water pumps and overheated engines so their stuff would tend to be overbuilt.
Spark plugs, go buy a set of the champions from the dealer, now go to a parts store and grab the same one from the parts shelf, compare the. I did and the OEM ones were different, good difference or bad difference I don't know but I do know that DC/Jeep has emissions standards to maintain while joe parts store is under no obligation. Just my .02
I also like OEM cause I know it fits, with AZ or others it's a crap shoot even if I have the part with me to compare.
Another from my industry, when I build servers or workstations I use full retail parts, CPU's and Motherboards also power supplies are top line. I sell my stuff with a 3 year warranty on all parts [hosed operating systems are the owners problem

and I charge for that]. Reason being that I can save $10 or so on a cpu from intel with it's OEM rating, warranty is DOA or 90days, Retail is 3 years. Is there a difference, can't say for sure but I know that alot of P3's and P4's that came out of oem plastic pacs came back alot more often then the P3's and P4's that were in retail boxes with intel fans and heatsinks, saving $10 on 40 units can be eaten up with the first two bad ones that I have to replace because the Intel warranty ran out plus shipping back to the customer.
I watched cheap machines get built and sold using cheaper parts and I watched them come back in 1-2 years later.