Gear lube for 4kq

[Andrew Buc]

According to the Bentley, the 4kq transmission takes SAE 80W or 80W/90,
GL4, and the rear diff takes SAE 90, GL5. I've heard good things about
Swepco gear lube (www.swepcousa.com), and today I spoke to their local
distributor. From what I could see on the website, it's supposed to be
a real premium product, w/price to match, even though it isn't
synthetic. The closest grade they make is 80W/90, GL5, and I wondered
if this would be OK. The guy said it would give at least as much
protection as SAE 90. As to GL4 vs. GL5, he said the GL5 rating
supersedes the GL4 and is better. I got the impression that this was
analagous to being able to use, say, SG-rated engine oil (I don't know
offhand what rating they're up to these days) in an older car that
calls for SF or lower. But I didn't explicitly pursue this line of
thought.

Pending getting feedback from the list, I checked Castrol's website,
figuring that they'd have a wide range of weights and grades. Well,
they have both GL4 and GL5, but no SAE 90--closest is 80W/90. If I go
with Castrol (Swepco is pretty spendy), will 80W/90 be OK in the rear
diff? Thanks!