Transmission Oil vs. Motor Oil? Can Cause Shifting Problems?

You’d think that changing your car’s transmission fluid — and filter, never change one without the other — would keep everything working correctly in Corolla’s automatic transmission but that is not necessarily the case, at all.

Your Corolla is now four model years old and if you do the average amount of driving (15,000 miles per year), it probably has around 60,000 miles on the odometer, give or take a few hundred. And, really, that’s not a heavy duty amount of driving, unless the area where you drive is humid and hot or really dry and dusty. So, it is really unlikely that just changing the transmission fluid would cause any major problems.

But here’s something to think about! It is possible that there is some metal to metal wear going on in the transmission. It is also possible that the same metal to metal wear is generating all kinds of debris that is circulating through your transmission. At the same time, it is possible that the filter is getting plugged up and losing effectiveness. Now, you’d think that just changing the fluid and filter would be okay and shouldn’t cause any problems, however, it is possible that the debris that was generated has caused damage to some sensitive parts like the bands or clutches, or, possibly the gearing. And, it is also possible that since the filter wasn’t as effective, it didn’t stop that damage from happening. So, it is entirely possible that just changing the fluid could cause a problem.

And, here’s another thought. Since transmission fluid is detergent because that is one of its manufactured properties, it is also possible that if there are any deposits built up in the transmission that they are being cleaned out by the newly changed, detergent in the new transmission fluid. If those deposits remain large, they can damage the transmission, causing further problems.

One more thing to consider is that you mentioned the transmission fluid was black both times it was checked. You indicated that the first mechanic changed the fluid but if it was black, again, after only 2,500 km of driving, you have to ask two questions: was the fluid changed at all, and, if it was changed, what is causing the fluid to turn black so quickly? Frankly, it isn’t likely that the fluid was swapped the first time — that is just my opinion, of course — and that is why the fluid remained black.