I use the following, they do comma ep80/90 GL4 and are the only folks local that I could find stocking a GL4 spec gear oil. Prices are good, something like ?20 for a 4.5l can I seem to remember. That may include a wee discount as I get it through my brother who works at McTaggarts.
Eskside Motor Factors 0131 6632129, Grannies Park Industrial Estate, 24 Edinburgh Road, Dalkeith, Midlothian, EH22 1JR
Martin.