Redline MTL vs MT90

[Dave Conner <conner@cfm.ohio-state.edu>]

What would be the difference between these two lubricants in the manual
transmission of a 1989 Audi 200 quattro?
Would I notice a difference?
Is one preferred over the other?
I have MT90 in it now.

But I get occasional clashing/grinding during shifting.

I have MTL in the transmission of my 1986 Audi 4000 quattro and shifting is
butter smooth.  
I don't know what was in the 4kq before I switched to MTL.  
But I thought the synchros were trashed... gears never wanted to mesh
properly.  
A friend with another 86 4000 Q recommended I switch to MTL and I couldn't
have been more pleased at the difference.
It felt like a new transmission. 
Dave C.