The fluid in the gearbox (even with O/D) can make a big difference in how it behaves. A "thick" or "sticky" fluid will hold the syncro-cones and the output gears to the shaft a little better keeping them "up to speed" with the drive-shaft (and hence the rear wheels). This requires a little more work be done by the syncros and dog-ring but the added "cushion" of the heavier oil softens this blow a little too. My worry with heavier gear oils has always been that the oil has to get into the plain bearings by splashing as there is no pump. Shouldn't be an issue with an old box but my personal belief is that heavy oil should be avoided in a new box unless the manufacturer specified it. Lighter oils (even ATF) will allow everything to spin more freely and require a better "feel" for the engagement or even double-clutching (heel&toe).

The O/D, on the other hand, works like an automatic transmission. Hydraulic pressure is shunted to pistons that engage the cone-clutch to cause the sun-gear to lock-up and give you the gear multiplication. The pump cares not a whit what is in there. Really, it is all non-compressible. Foaming is the enemy. The pressure developed (in a properly sorted unit) will be the same. Now the bleed orifices and valves will see some restriction of flow with a "heavier" or "sticky" oil and a very light oil will seep past seals and orifices easier making the line pressure at the engagement pistons vary. Big difference to me with oil choices is the behavior of the cone-clutch. I don't want to have that assembly have to force extra thick oil out of the way in order to lock up (with associated heat build up and potential burning of lining or oil). Perhaps this system is also self-regulating to a certain extent... heavier oil causes more line pressure due to bleeding off slower so it can force the (also) heavier oil out of the cone more quickly.

All of which provides a good reason to not engage the O/D under power.

Gunk in there will mess it up far worse and more quickly than the "wrong oil" in my opinion.

What is right? Whatever works for you. I have heard of everything from hypoid GL-spec gear oil down to ATF and people will swear up, down and sideways that what they use is "proper" and "the best."

What do I use? Straight 30W (detergent, I just clean the screen more often).

Cleaning out the system is the best first step. If the box had heavy oil in it before and has light oil now I suspect a lot of debris got loosened up and is in your O/D sump and bottom of gearbox.

Mike!