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Q: oil changes

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I am currently using synthetic oil..Would it be safe to use regular oil after the next oil change? I have been told so many things, I just want a true mechanics' advice..

My car has 95000 miles.
My car has an automatic transmission.

Tribologists, who are employed by companies that refine and sell lubricating oils, as well as the engineers at the car engine manufacturers, are the individuals who determine best practices when it comes to specifying engine oils. Mechanics merely convey their advice to the public as mechanics do NOT have the necessary laboratory data, test data and/or engineering background to give authoritative advice on lubricating oils. The fact that you have gotten conflicting, inconsistent advice from mechanics reflects that fact (i.e., mechanics repair cars and trucks; they do NOT perform laboratory or field testing of engine oils, period). So, to emphasize, you are asking a scientific or engineering question and the applicable science is, in part, Tribology. To answer your question, which is a good one, according to lubrication experts (Tribologists), with only some minor caveats as stated below, synthetic oil can be used at any time, and you can switch back and forth between conventional oil and synthetics at any time and with any frequency, with no negative effects. Although existing leaks may potentially leak at a somewhat greater rate when using synthetic oils versus purely mineral based oils (aka, "conventional" oil), the added leakage, if any, is meaningless in most seal locations and in most circumstances. The potential for a leak with synthetics is due to the smaller size of the molecules in synthetic versus conventional oils. However, synthetic oil cannot possibly cause or otherwise mechanically enlarge a leak. In any event, any "distinction" in leak rate is meaningless simply because if you put synthetic oil in a car and you can see a leak from a seal, that seal was most decidedly leaking anyway with regular oil and so would have to be repaired regardless of the oil "type" you are using.


In your specific case, with a relatively newer car and moderate miles on the engine, this possibility of a leak with synthetics probably does not exist at least for a while, although you are getting close to the time frame when just due to rubber aging (over time, not just mileage related), leaks will start developing anyway. Although synthetic oils are always preferred due to their advantages in physical properties and the potential to lengthen the service life of your engine and even seemingly unrelated parts such as oxygen sensors and the catalytic converter (synthetics don’t have the sulfur and other elemental contaminants that mineral oil has), you will not damage your engine by using regular oil or by switching back and forth between conventional and synthetic oil. Full synthetics will be less subject to degradation and evaporation during the oil service interval, too. Changing your oil removes dirt and contaminants and newer cars with all sorts of oil actuated mechanisms, such as variable valve timing, need VERY clean oil. YourMechanic offers oil and filter changes during mobile visits right to your location and you should certainly avail yourself of that service as the cost is lower and the service is much more personalized than at a shop or dealer. If you have additional concerns or a follow-up on this, don’t hesitate to re-contact YourMechanic.

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