Some background info is in order, if you are interested.
Many think all gear lube is the same, as long as you pick the right weight. Even many mechanics pay no attention to anything other than weight. Well, wrong, although it used to be true. Unlike motor oil, gear lube is indeed made different for different applications, even for the same weights, as newer cars requirements are so different from older cars.
Mistake one: I had a pro shop change the gear lube in a manual tranny pushing close to 200k.I figured it was time :) If you want it done right, do it yourself, but I was lazy that day. I specifically requested them to make sure the right synthetic fluid went in. They of course put in the right weight: high priced, major brand, synthetic 75w90, but they never bothered to check anything else, since they just have the autostore cross-reference and deliver the fluid. The stores are more clueless than anything else. The chain stores do not even stock GL-4 fluid anymore. There was one high priced brand that claimed to be both GL-4 and GL-5. Well, anything that claims to be one size fits all usually does not fit anything properly so I would stay away from that.
Mistake two: Not checking if all lube was backward compatible. If you want to be sure, check everything yourself. After all, I had professionals do it for me, right? They don't care because it is not their car. Well, GL-5 is not backward compatible and can actually damage some tender parts in transmissions designed for GL-4 due to some harsh additives.
So, after the shop put in GL-5, after a week or two, tranny started lurching in 5th gear. Darn. Did some research, but could not be sure if it was the fluid or just coincidence. After all, it is a lot of miles on the tranny. But, all indications were GL-5 was bad for my GL-4 designed transmission.
So, all my hopes rested on this Redline fluid. It did not let me down!
Got some Redline MT-90 GL-4 75w90 fluid and changed it myself. Amazon has the best price. No one locally even stocked pure GL-4. At first, the lurching was still there. Well, it was worth a try, I thought. But then, it got smoother, or so it seemed. Wishful thinking, I thought. Then, it was definitely smoother, no mistake. Could it be? Now, about two weeks after, for the very first day, the lurching in 5th gear is completely gone, and the tranny is as smooth as it was before. Even smoother. I would occasionally grind when shifting quickly, but now I can shift as fast I am capable of and I have yet to make it grind.
So, expensive lessons learned.
One, do it yourself, if you can. Check what someone else puts in your car before you let them put it there. Definition of a "professional" is that they are paid to do it, not necessarily that they do it well. Just because they get paid to do it, does not mean they know what they are doing, or know better than you.
Two, even if you do not do it yourself, at least make sure your mechanic knows what to put in. They will not take time to research and could not care less if you do not tell them beforehand.
Three, do not trust clueless autostores to cross-reference fluids. They will get it wrong most of the time. Go to factory service manuals or other reliable sources for that information.
Four, if your transmission calls for GL-4, do not chance it with GL-5! Brass synchronizers cannot take it, as GL-5 additives can eat them. You may need a new tranny or major repairs before you know it.
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I've been a longtime Redline lubricants fan because of their high quality and specific properties of the products themselves. In this case, (MT-90) the key property is to provide enough friction between gearshifts to allow the synchronizers to slow down and provide smooth gearshifts. This is not an easy task to perform while still lubricating and being slippery enough to provide a consistent oil film, lubricity, resistance to thermal breakdown and long fluid life. If you have an older transmission that's getting more difficult to shift, Redline MT-90 can improve it and extend the time before a rebuild is necessary.
