Oil question

[JM Dion <dion.longpre@sympatico.ca>]

Did anyone hear that SAE 80w90 is too heavy for a Triumph Tranny and
could cause early failure ?  A local English parts dealer told me that
there is a mistake in the books who recommand SAE 90 (but I have never
seen anything else). He suggested to use regular engine oil like 20W50.

Thanks

Jean-Marc Dion
MKII 1967
Quebec, CDN