[TR] Proper oil for TR4 Gearbox

[Randall]

> Though I think the weight difference is small -- isn't 90W 
> gear oil about the same viscosity as 40W motor oil?

Approximately, yes.

TR4 owners handbook calls for 90 weight GL4 above 30F; 80 weight below 30F.

I've been running Valvoline VR-1 20W50 motor oil in the gearbox Herman van
den Akker rebuilt for me (TR6 gearbox, TR4 OD, TR3 top cover), because that
is what he recommended.

Randall