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commander Dave

mixing engine oil

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Time to change oil in the big 6. I'll use Valvoline racing oil and want to use 40wt.

Will it hurt to use some 30 or fifty wt. of same brand in with it?

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It might, why not just use 10-40? If it was my lawn mower engine I would not be worried.

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I've always heard it was not a good idea to mix different oils. The term I'd hear was it would "sour the oil". Maybe it was because of different additives one oil would have and others would not? The only thing I can think of is that there are oils with detergents and some were without and that might be the problem.

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How is your oil pressure when the engine is fully warmed up running at low idle? If it goes below 15 psi I would not go to a multi viscosity. Non detergent oil has not been recommended for vehicle engines since about 1930 but people still used it for years. Non detergent oil is less likely to break any oil sludge build up that may be in your engine but it will not contain some of the newer additives that are beneficial for longer engine life. I started running a model T 4 years ago ( it was setting for 15 years or more) and I put 15w-40 in it and it seems to run well and no unusual noises or signs of low oil pressure trouble. I just started an EW about 2 months ago and with 15w - 40 in it the oil pressure warmed up at low idle drops to 10 - 15 lbs. I just changed it to straight 30 weight but I am doing other work on it so I cannot tell you if it helped or not until I get to run it again. Oils years ago were very different bases and some were from a paraffin base, those oils I would not mix or even use if I found a deal on some today. I would not be afraid to mix current name brand oil with the same name such as Castrol ( Brand ) GTX ( name ) of different weights.

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I have heard a couple of places that the detergent additives in different brands of oil USED to be quite different and possibly incompatible. So the safe thing to do was not mix brands. I bet when they started doing API service grades that problem went away (if it ever really existed). Thing about it, when people need a quart they buy whatever they can get and dump it in. If that caused engines to grenade up though the hood we probably would have noticed it by now.

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From Commander Dave; Changed oil in big 6 using Valvoline racing oil using 40wt.

.Have noticed that in summertime and after about 10 or 15 miles the oil psi can drop to 15 lbs.or slightly lower while driving and less at idle. however, in moderate temps it stays around 25psi.It smokes and has a fair amount of [blow bye].I know it needs rings but is this oil pressure within normal range?

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I am not sure about the Commander engine but for the Champion the book says you should have 40 PSI oil pressure at 45MPH in high gear. Mine runs just about exactly that. Pressure drops way down at idle. It burns a little oil but has very good compression so I assume valve guide wear.

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Problems with mixing oils are due to different formulations. You may be degrading the oil with a lower grade of oil. It is from the same stock? Does it have a different formulation? Is it the same API rating? WIDMAN.biz has a calculator to show the net viscosity when you combine oils. Does each oil have the same level of additives you are looking for?

Multigrade oils: the first number (e.g. 10W) is about viscosity when COLD. It has little to do with viscosity when hot but of course does affect the viscosity as the engine warms. If you want minimum wear on startup (when most wear occurs), make sure the thermostat works at the right temperature and use a 5 or 10W oil (or 15W if it is very worn or a very hot place). The second number relates to viscosity when HOT. So if you want a 30 wt oil, use a straight 30 wt (I wouldn't, too much wear when engine is cold), or a 5 or 10W-30.

BTW, www.widman.biz has a good treatise on oils for flat tappet engines.

Edited by Spinneyhill (see edit history)

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