View Full Version : mixing regular and synthetic gear lube-- problems?

jim davies
05-12-2008, 01:03 AM
Anybody ever mixed synthetic and regular gear lubes and had problems? Just wondering...

05-12-2008, 06:18 AM
New outboards come with a petroleum based gear oil in the gear case. After the first 20 hours of use, I drain them and put a synthetic gear oil in. I don't flush the gear case and there is always some residual petroleum left that mixes with the synthetic. I've been doing it this way for about 5 years and never had a problem. I don't think however, that mixing larger amounts of the two would be a good thing, even though most synthetic gear oils are a blend and not 100% synthetic.

05-12-2008, 07:18 AM
As always revert to manufactures recommendations.

Ian B
05-12-2008, 07:22 AM

We run several gas turbines offshore, and have changed over to synthetic lube oils. We checked with the lube oil manufacturers; they told us that regular and synthetic can be mixed in any ratio without problems - just that the mix will be less effective than pure synthetic (but better than regular).

No problems with the oil turning to a gel or whatever - I think that was only when adding mineral oil to a vegetable oil like castrol R...


05-12-2008, 07:43 AM
Shouldn't be a problem these days. Some early synthetics weren't compatible with petroleum, but the ones you get now generally are.

05-12-2008, 11:43 AM
My understanding is that the term "SYNTHETIC" doesn't mean that the oil is created from something other than petroleum, but rather that it is perhaps hyper-refined and then the components are put back together for a specific application. You are still mixing oil & oil. The only thing I have ever heard of where there could be a problem would be from mixing parafin based (Pennsylvania Grade) oil with acid base oil (most of the non-Pennsylvania Grade). Other than that oil is oil. Unless you are really pushing the limits of the lubricating or heat transfer properties you will probably never see any difference.