Redline 75W90NS or another transmission oil eg Valvoline 80W90?

[Simon Body <body@domain.elided>]

Hi

I plan on replacing the diff and transmission oil on my 1991 Spider this 
week. Both are working reasonably well although I get some grinding going 
into first and reverse. I've recently bought the car and am not sure when 
either fluid was changed.
The question: Shall I refill with Redline 75W90NS (the expensive, difficult 
to obtain stuff) or with another GL-5 rated transmission fluid that I can 
get 5 minutes down the road for half the price?
Thank you

Simon Body

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