"Old" engines or cars are anything from before when you were born
. But seriously, I've anecdotally observed it seems like problems when switching to synthetic oil come up with major changes in base weight; say, going form 10w-30 to 0w-30 synthetic because the latter really is more stable at higher temps but supposedly without the need for as many viscosity index enhancers or modifiers even if both have a rating of "30". But, the thinner base stock ("0" rather than "10") might make for more leaks or oil pump cavitation if oil with a "0" wasn't originally specified by engine manufacturer for anything above cold ambient temps. Our auto repair shop recommended continuing with 10w-30 for both our '03 Jeep Rubicon I-6 and a '97 Ram van-based Roadtrek with the 5.2 Magnum V-8 whether it was synthetic, or not. We used synthetics there with no problems. And, even Ford had previously recommended 5w-20 synthetic or blend on a '87 and '97 Mercury Sable with the 3.0 Vulcan V-6 that originally called for 5w-30 non-synthetic.
So, check with manufacturer or a mechanic who keeps up on this.
Remember, the oil is all hydrocarbon-based. Synthetics may be derived from heavily synthesized natural gas or crude oil; non-synthetic is derived as distillate from crude oil. The former may be better simply because it's refined and reconstructed exactly as the manufacturer wants it.
Edited by michelle_east_county, 11 October 2021 - 07:36 PM.