Dear Tom and Ray:

My friend drives a mid-1990s Jeep Wrangler. He took it in to get serviced a few weeks ago. The service included changing the transmission oil. After the service, he noticed that the shifting had become rough. Because the roughness didn't go away after a few weeks, he had his mechanic friend take a look, and the friend was able to tell by the color that the transmission oil was 80 weight and not the recommended 90 weight. My friend called up the shop, and they claimed to have used 80-90 weight. What damage can an 80-weight transmission oil cause to a transmission for which 90-weight oil is recommended? -- Franklin

RAY: Gee, Franklin. I don't think it would make any difference. And all the transmission oil I've ever seen is the same color -- ugly dark brown.

TOM: In fact, we don't even see straight 80-weight transmission oil anymore. Everything we use is multi-viscosity -- it's either "80-140," meaning it performs like an 80-weight (thinner) oil when it's cold and like a 140-weight (thicker) oil when it's hot, or it's "80-90," which is what this shop claims to have used. And that should work fine.

RAY: I can only think of two possible explanations for your friend's problem (other than that he's imagining it, or it's a complete coincidence). I suppose that if they accidentally used the 80-140 oil, it might be a little thicker overall and might slow down the synchros enough to make the shifting feel a little stiff or notchy -- especially if there was an existing problem.

TOM: The other possibility is that they accidentally put automatic transmission fluid (ATF) in your transmission. That's a got a red color, and since it's considerably thinner than 90-weight oil, it might make the gearshift feel kind of "loose."

RAY: I'd say, in either case, for your friend's peace of mind, he should go back and ask them to drain that stuff out, and make sure they fill it back up with 80-90.

TOM: But to answer your question, it's unlikely that any permanent damage has been done.