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Help! Engine oil in Automatic Transmission!

Ok I did a stupid thing last night. I was checking the automatic transmission fluid on my Toyota pickup (181,000 miles) and I needed some. Unfortunately, I grabbed the wrong bottle in the growing darkness and poured 5W/30 motor oil into the funnel. I noticed it instanly but it was already to late and about 1/4 cup of oil got into the transmission. The engine was running of course since I was checking the fluid level but I haven’t cycled it through the gears or driven it. I’m planning to drain the fuild tonight and drop the pan to get as much of the oil out a possible but I’m worried about any damage or issues I may experience because of the contaminated transmission fluid? Thoughts?

Dont worry, engine oil will not damage an automatic transmission. You dont need to drain it unless you really want to. I would just let it go but do whatever makes you feel better.


Yeah, don’t worry about it!!

Not to worry. Transmission fluid is just 10 weight oil with the proper additives for that function. That little bit of 5W30 oil won’t do a thing.


You can actually do the opposite and do some good for your engine, the big difference between ATF and motor oil is a detergent that you can find in the ATF, the detergent is to keep the interior clean, because of fine metal shavings that you get in your transmission, which is why the plugs are magnetic. A little motor oil in your transmission isn’t going to do anything to it, just keep your transmission lubricated, that’s all.


Thanks for the advice guys. Just as a precaution I went ahead and drained the fluid and dropped the pan to clean out as much of the motor oil as I could. I refilled and she drives fine this morning with no issues.

Afterward when I was draining my catch pan into the waste oil jug I could see a bunch of the motor oil separated on the bottom of the drain pan. The transmission may have been fine leaving it in there but a few bucks in ATF, a couple hours and your advice gives me more peace of mind.

Thanks everyone,