Anybody tried using engine oil in the Sierra diff to reduce friction for a BEC car - that ep90 is like treacle!!!

...on whether the diff. is a slip diff or not. If it is LSD IMHO I wouldn't use anything other than recommended oil.

Do bear in mind however, that diff will not be doing as much hard work in a BEC. Would still use recommended oil though. HTH

the gear oil is heavy to resist the pressures that the gears meshing
together generate if you use engine oil the diff will be fubar very quickly