Easiest way to check refill oil in transmission?

[diogenes]

Yes (on a level surface).

And this may spark debate (and I may be wrong here) but I've been told to use Hypoid 80/90 GL4 not GL5 when refilling.

That is correct. The reason is that GL4 is compatible with "yellow metal" e.g brass, copper, bronze that is used in our older vintage cars. GL5 is formulated for newer cars and apparently not "friendly" with those materials.

Our club's lubrication guide for the transmission lists only Red Line Synthetic MT90 75W90. The bottle says "GL-4 High-Performance Lubricant for Manual Transmissions and Transaxles. Not Designed for Hypoid differentials"

For the differential: Sta Lube GL4 Multi-purpose Hypoid Gear Oil; Castrol GL4 or Brad Penn GL4. Brad Penn bottle is where they mention "yellow metals" which we have in our differentials. The standard replacement thrust washers are copper. I know that for a fact because I just had mine replaced last month in my '74 TR6.

Summit Racing has both.