Mareo you need to get your facts straight, thicker isn't necessarily better.
For the record, for those who will question the validity of what your about to read, I got my first 1200 in 1973, I have been working on cars and trucks professionally since 1975, and I spent more than my fair share of time working for a Datsun/Nissan dealer, and teaching others mechanics.
Heavy gear lubes work great in differentals, and ONLY differentals, PERIOD. When thick oil is cold, it will not flow and will starve the bearings for oil, manual transmissions have needle bearings in them that require lighter oils to propperly lubricate them, thick oils, until they get hot will not flow. more to the point, and think please before you get mad, how often on a cold morning do you have trouble shifting your trans? this is because cold thick oils are by nature sticky and hard to stir. EXAMPLE take a small amount of 90W gear oil, put it into a cup and then put it into you freezer for an hour, after it's cold, try to pour it out.
With this in mind, think about how well this same cold gear oil is lubricating the inside of you transmission. it doesn't take much to see that it don't work too well, does it?
more to the point, almost all new car manufacturers now recomend using a lighter oil in thier manual transmissions,this includes for replacement in older models, some have gone to the point of making it a pollicy of voiding warrantys if anything other than motor oil or ATF are found inside thier transmissions. WHY? simple, lighter oils flow better when cold and they can handle the heat when hot. transmissions almost never see the heat that is generated inside of a running engine.
Before anyone slams me for this, think about it, I can back it up with facts, not myth.