I just took my car in for an oil change and decided to get the high mileage oil change which involves a synthetic blend for cars with over 75k miles.
I have read that in older cars synthetics are a bad idea because oil gunk from conventional oil forms seals around potential leaks and synthetics dissolve these seals and cause oil leaks. But when I got the oil changed I didn’t know the high mileage oil was a synthetic blend, I thought it was conventional.
I’ve also heard not to get a transmission flush on an older car for the same reason, the gunk may be holding the transmission together. That is unrelated, I’m bringing it up because it is somewhat the same concept (cleaning out an engine or transmission could remove gunk that is stopping leaks).
I’m not a mechanic though, but now that I’ve got a synthetic blend in my car do I need to use synthetics from now on or will conventional suffice?
If I do start to get oil leaks, what do I do?
Is this concept of synthetics causing leaks true? Would a synthetic blend designed for higher mpg cars do the same thing?
I’ve never subscribed to either notion. as far as the oil goes, if you’ve got enough “gunk” in your engine where its presence makes the difference between leaks or doesn’t leak, you have way bigger problems imminent than what kind of oil you buy. I have heard that synthetic oil is not as likely to thicken/increase viscosity as it ages so if you do have existing leaks they may seem to get worse.
as for trans fluid, I think that myth has maybe a couple of origins. First might be people not understanding that correlation =/= causation, and thinking the recent fluid change is what killed their trans (not realizing it was near death already.) The big thing that fluid quality would immediately affect is the slippage of the clutches; and again if they start slipping shortly after a flush & fill they were already practically burned up. The second part might have been with the now infamous Chrysler 4-speed transaxle. They were buggy the first couple of years, but after that they were fine. However, they required a specific type of fluid called ATF+3 (later ATF+4.) At the time, many shops only had the typical DEXRON/MERCON stuff on the shelf, and would do a flush and fill on a Chrysler transaxle using D/M as the replacement. Lo and behold, the transmission would start acting up shortly after that owing to the incorrect fluid.
If you’ve been using good modern conventional oil, going to a synthetic shouldn’t make a bit of difference in loosening sludge or leaks. The system will be kept plenty clean with a conventional oil (changed properly, of course), and would leak with either type.
I think these myths date back to the use of non detergent oil…which would let sludge, etc build up. Suddenly switching to a detergent oil could possibly lead to “breaking crap loose” problems. (the detergent in oil was used to clean up and keep crap in suspension, so it would be filtered out)
But non detergent oils haven’t been used in vehicles for decades, so it’s not an issue here.