[Vwdiesel] Transmission fluid draining

[Roger Brown]

paulr wrote:
> I don't know if this is an "old wives tale", but I
> think I read somewhere that the additives in GL-5
> attack the (bronze???) synchronizers in the tranny????
> 
> Paul

That was probably true when GL-5 oils first came out and in older transmissions.  Most "modern" transmissions can take GL-4 or 
GL-5 oils and usually both are spec'ed.  The sulfur and phosphorus extreme pressure additives in GL-5 oils could attack the 
"yellow" metals in the gear box (brass and bronze).  I think once GL-5 oils became prevalent, most mfgs. changed the yellow 
metal alloys to withstand the additives.

The main problem with GL-5 oils is that they are too slippery, since they are designed for use in sliding friction applications 
like helical cut ring and pinion gears.  For a manual tranny, the synchro rings rely on friction/drag to spin the gear and shaft 
they are on up to speed to synchronize with the mating gear.  A GL-4 oil is designed for straight and bevel cut gears where the 
load is mainly a straight on contact as the gear teeth mesh.  A GL-4 oil is not as slippery as GL-5 and that lets the synchros 
spin up to speed faster.  So, while a GL-5 oil will not destroy a manual transmission, it will work a lot better with a GL-4 oil.

-- 
    Roger