86 CGT Transmission Oil

[Jeffrey A Blanchard <jblancha@u.arizona.edu>]

Hi fellow Qlisters,

I recently replaced the transmission oil in my 1986 Coupe GT with
Valvoline Dura-Blend (synthetic blend) 80W90 which has a GL5 rating, but
is recommended for GL4/GL5 applications (according to their homepage).  My
Bently specifies 80W90 oil, GL4.  I don't know much about gear oil
ratings, etc. but it seems to me that I picked the right stuff.

The car shifts fine when warm, but it now shifts terrible when cold. 
There is always some grinding between gears unless I shift really slowly. 
I don't remember this happening before I changed the transmission oil. 

Any suggestions?  If it continues, I plan to switch to Redline oil since
it is spoken of very highly.  Which type should I use?

TIA,

Jeff Blanchard
86CGT