ok so i have a 96 12v and plan on servicing both my trans and rear diff next week. when its cold my car shifts hard into second, doesnt grind at all but it pulls in hard. once the car is warmed up it shifts perfectly fine. the last time i changed it I put mobil 1 full synthetic 75w90 in it. what is the difference between the gl4 and gl5 fluid for these applications? is there a better fluid to run in these transmissions and differentials?

 

It is best to stick with the VAG specification. The Bentley workshop manual contains a list of the recommended fluids.

The difference between GL4 and GL5 is the amount of pressure that can be applied to two surfaces without loss of lubrication. When parts meet, the 'squeezing' that takes place forces the fluid out resulting in a loss of lubrication.

 

Don't use GL5 in your transmission, it has additives that eat your synchros over time. It's nearly impossible to find fluid that follows VAG specs in North America, I used Amsoil 75w90 and it's great except for the first 5-10 minutes of driving in winter when it feels like the shifter is in a box of jello. Even though VAG says to use 80w90, their 80w90 is actually closer to 70w90 in viscosity. If I were to do it again I'd use redline or motul 70w90.

For the rear diff use GL5, forget the viscosity but it should be easy to find.

 

If you are unsure about fluids available where the vehicle is operated, you can always go to an Audi agency and purchase the fluid sold for gearbox use.

You will need about 3 litres for the gearbox.

 

I use Redline MT90 in the transmission and Redline 75w90 gear oil for the rear diff.

 

I tried that at first, they wanted something crazy like $35 a litre, screw that.

 

There are many choices for fluids, purchasing the fluid from the manufacturer is one of them. As long as the fluid of choice meets the specification of the vehicle manufacturer, price comparison is another way to make your choice.

 

I did my research, wasn't able to find a single gear oil that meets the VAG spec sold here. Maybe it's easier in the U.S but it seemed that fluids that met the spec are only sold in Europe. The shop I bought my gear oil recommended redline mt90 or mtl, I should've listened to them since they warned me about how thick it gets when cold.

 

You can certainly change the gearbox oil to something more compatible with the conditions where the vehicle is operated. I have no experience using the redline product and can not speak about any affects it may have on shift quality or reducing internal wear.

I am curious about what additives are included in GL5 fluid that you found can corrode the brass blocker or synchroniser rings.

 

I personally believe the whole GL5 thing about attacking brass is a myth. it may have been true years ago, but has been handed down now for years.