Hi Ally,
Its true you shouldn't because the GL5 stuff because it has some nasties in it which destroy the copper washers in the diff. It also mentions this on
another site which has all the specs for Spits:
http://www.triumphspitfire.com/Fluids.html
"It is essential to use an EP (extreme pressure) oil to GL4 spec only for the diff and overdrive. DO NOT USE GL5. Virtually all oils now sold are to
GL5 spec and will "melt" the copper washers in the diff. 80W90 GL1 is another great alternative. The GL1 can be found locally at Tractor Supply or farm
stores.
Mineral oils are usually EP90, semi synthetics etc can be EP75W90."
I know that Halfords do GL4 hypoid oil as I had to buy some when mine was leaking. Its probably better to be safe and buy some - I think its about 6 or
7 quid for a litre. You can always use the GL5 stuff in the gearbox anyway ; )
Hope that helps, Umesh
