Several times I've posted relating to my "success" substituting 80w90 gear oil in my M410...how it cured the cranky overdrive.-
But I've recently become aware that the GL5 additives in most all readily available gear oil may be a no-no for older transmissions. This was confirmed when I called Dave, the tech guy, at RedLine Lubricants.

They make a dedicated manual transmisison oil, MT-90, that he felt would work fine in the application (my car is probably older than him, so I'm not sure that wasn't blowing smoke) and actually improve shifting because they leave out the friction inducing components that gear oils have, allowing the sunchronizers to spin up faster. Hey. That's what he said. I don't claim to fully understand it but they do treat the subject on their web site.

Ordered some from a supplier (Ouch, it's salty). Tranny and overdrive take about two quarts, but I always spill some so I got a gallon. After a couple of hundred miles I can heartily endorse the product. Overdrive still works fine, and it actually does seem to shift a bit better. That may be wishful thinking.

But, regardless of the performance, I feel better knowing my synchronizers are not being eaten away by those pesky GL-5 additives. Hope anyone who heeded my advice about regular gear oil will research this further.