Is there a problem with mixing synthetic and conventional oils?

Dear Car Talk

Dear Car Talk | Jan 01, 2002

Dear Tom and Ray:

I recently purchased a 1995 Mitsubishi Mirage. The previous owner bragged that he used only "the best" oil in it, and he showed me an empty quart of Mobil 1 synthetic oil to prove it. I bought the car because it was in good shape and had low mileage, and I couldn't have cared less about the high-tech oil ... until now. It's time to change the oil, and I really can't see spending the extra dough on the fancy stuff. However, the guy at the auto-parts store told me that once a motor has synthetic oil in it, I have to keep using it (unless I "flush it"). So I went to the Mobil Web site, and according to it, Mobil 1 is compatible with conventional oil. Can I switch back? -- Bill

TOM: You have the right to life, liberty and the pursuit of any damn motor oil you like, Bill.

RAY: There is no problem we know of with mixing synthetic and conventional motor oils. I think when synthetics first came out 25 years ago, some manufacturers weren't sure how well they'd work, whether they'd eat seals and gaskets, or whether they'd mix with conventional oils and create cement.

TOM: But those concerns were largely dismissed long ago. For some reason -- we don't know why -- there are still a few manufacturers that warn against using synthetic oils and some that warn against mixing other synthetics with THEIR synthetic oil. We've never seen a problem in the shop related to this stuff, but you should check your owner's manual for a prohibition, just to be safe.

RAY: But the vast majority of manufacturers now treat synthetics just like a premium, high-priced motor oil that can be mixed and matched with other oils at the whim of the customer -- that's you, Bill. So as long as your owner's manual doesn't specifically forbid it, do whatever you want.

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