So what exactly is gear oil? I know it has to do with the gears in the transmission but when and how do you go about changing it? I bought my car with no records and had the tranny fluid looked at today...my mechanic told me it was brand new looking. What do I need to know about the gear oil tho? Was it changed when the tranny fluid was changed?

 

Gear oil is the trans fluid.

 

its thick, like 85W-90 or so, when motor oil is like 5w-30. If your mech says its brand new it likely is, as he'd like to make some $$ changing it for sure.

Two things though, when you do change it, I would use synthetic, and more importantly, make sure its GF-4 not GF-5, trust me on that one

 

So Gear Oil is the same thing as Transmission Fluid? Just trying to clear things up.

 

Automatic transmissions use transmission fluid. Manual transmissions use gear oil.

 

Good to know. Thanks for clearing it up.

 

*Most manuals use gear oil, some use ATF (automatic trans fluid) See older saturns and chevy S-10s for examples

 

Whichever you get make sure it is cholesterol free and has no trans(mission) fats.

 

They are two different scales.
http://en.wikipedia.org/wiki/Gear_oil

If the existing gear oil is clean, then it's safe to leave it alone. Old gear oil drains black as night.

Dave

 

Yes my mechanic told me that it looked brand new. Ill leave it for 25K miles and check it then. (unless the tranny starts acting up....)

 

GF? you mean GL.

synthetic is only better under certain circumstances. And what kcryan means is: get GL-4, not GL-5. GL-4 is older and doesn't contain the harsh detergents that come in GL-5 gear oils. Our trannies have brass synchros that get eaten by these detergents. GL-4 oils don't contain them.

 

I don't think gear oils contain detergents. There are no combustion byproducts to keep cleaned.

They do contain EP (Extreme Pressure) additives. These are chemicals that bond to the surfaces of the gear, leaving a microscopically thin layer of softer material. In moments of extreme pressure, where regular gear oil would be completely forced out of the region between the metal, it actually causes some of the metal surface to break free with lower friction than without the additives. The result is that the steel gear teeth actually polish themselves to a better finish and in things like hypoid differentials, the gear wear is much improved, and the lower friction significantly reduces breakage.

The problem is that what EP additives do to help steel gear teeth wear happens also to synchros, and usually way too aggressively. Synchros rely on metal-metal contact to perform their job, and it doesn't take long before the polishing action of the EP additives leaves your synchros with no bite. While you can buy a GL-5 that is chemically more friendly to the brass synchros, the frictional properties still put the synchros under more load than a GL-4 would.

How much all this really matters can be debated endlessly. Nissan officially recommended GL-4 only for a long time, but the dealers argued that getting GL-4 was too difficult and made no difference anyway. (After all, the tranny just needs to survive the warranty period, and poor shifting isn't exactly easy to make a warranty claim on). So now Nissan dealers can and do use regular GL-5 in all manual trannys.

IMHO, clean gear oil is most important; so change it every 30 or 60k. If you intend put in synthetic to improve shifting and DIY, take the extra effort to source Amsoil MTG (GL-4) or Redline MT-90 (GL-4). It costs about the same as the other synthetics and you can rest soundly knowing nothing chemically bad is going on in there.

Dave