To mix or not to mix is an old question that keeps surfacing

Bob Sikorsky

Originally published Saturday, April 13, 2002

Dear Bob: Recently a reader asked whether different brands of motor oil could be mixed. You responded by saying that oils could be mixed as long as they carried equivalent API ratings.

It is my belief, admittedly based on 30-year-old information, that you should not mix different brands of oil because they fight each other at the molecular level, which then creates unwanted friction and wear in the engine.

Can we really mix discount-store oil with a premium synthetic, or does my antimixing argument still make sense? -- Kenneth Markunas, Troy, N.Y.

Answer: Ah, nothing like 30-year-old information to stir the oil pot.

Usually, when people ask about mixing different brands of oil, they are referring to topping off their current brand with a quart of another brand when they are a quart low. There is no harm in doing this, so long as the API service rating of the added oil is equal to or higher than that of the oil in the crankcase.

For example, if both oils are rated API SL, these oils won't fight each other, nor will they create unwanted friction. Take that, you 30-year-old information!

Don't take my word for it. In a recent article in National Oil & Lube News, Mike Maddox, national manager of technical services at Pennzoil-Quaker State, talked about this very subject, and concluded by saying that " ... most major brands of motor oils are typically compatible with each other."

So while the maker of your current oil would doubtless prefer that you stick with the same brand, it won't hurt a thing if you have to add a quart or so of a different brand, so long as it carries an API service rating equal to or higher than that of what you've got in the crankcase.

Dear Bob: I have a 1985 Volvo with 140,000 miles in mint condition, which I drive only 3,500 miles a year. It has developed a small leak in the rear engine/transmission seal, about a couple of drops overnight.

I know there are additives that are supposed to swell the seal and stop the leak, but experts I talk to are divided over their use. Some say that they can do more harm than good, while others say that they're fine and will help. With a leak of only a few drops per night, I'm not about to expose a 17-year-old car to the trauma of having its transmission removed to replace a seal.

Would using one of these additives help avoid future transmission surgery, or would it put my engine/transmission at risk? -- Peter Stora, Bellerose, N.Y.

A: Seal-swelling additives are a double-edged sword. On the one hand, they will help stop a leak, especially a small one such as yours. On the other hand, the fix they provide is usually of limited duration and, once the seal-sweller becomes ineffective, you will have no choice but to replace the seals. The seal-sweller will have inflicted permanent swelling damage to them, and the leak will be a lot worse than it was before you added the seal-sweller.

With such a small leak, my advice would be not to add anything at this time. If the leak becomes more serious, however, you might consider a seal-swelling additive, knowing in advance that it is a last-ditch attempt to keep the seal-leak wolves at bay. After the seal-sweller has been added and the seals start to leak again, you will be looking at a seal-replacement job.

Bob Sikorsky writes for The New York Times Special Features syndicate. Address questions of general interest to him c/o The Olympian, P.O. Box 407, Olympia, WA 98507.

The Olympian Copyright 2002

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