The Laycock manual recommends Mineral oil SAE 30 or SAE 50 for the overdrive. Is this still the recommended lubricant for overdrives?

 

I found some post that stated 30 motor oil should be used in the Moss 4 speed non-synchronized 1 gear transmission.

 

Careful. What overdrive gearbox are you talking about?
The Moss box jayd2 is talking about definitely should use SAE 30.
The later all synchro box should use SAE 90.

The easiest way to tell these boxes apart is the reverse is to the left and forward in the Moss box.
In the all synchro box it is to the left and back.

For more specific information on oils we can expect guidance from Glyn.

 

Bill you basically have it spot on. The only guidance is that the gearbox oil that is circulated by the OD pump should be GL4. GL5 will damage your yellow metal synchro cones.

 

GL5 lubricants are made for differentials. In today's multigrade era I use 75W-90 in the gearbox/OD and 85W-140 in the differential.

A 75W-90 will give easier gear changes while the oil & gearbox are cold.

 

I'd definitely avoid classic, multi-grade, mineral engine oil. After it's chewed up by the gearbox, it can become quite thin and a well worn overdrive might not have enough pressure to engage after a fast drive on a hot day.

 

There is a straight SAE30 oil (non multigrade) which is recommended for Briggs and Stratton four stroke engines.
It is readily available at mower shops and larger auto stores.

 

So called "Classic" oils are absolute junk & use rubbish base oil and the cheapest non stable VI Improvers on the market. Then they have the cheek to charge you a premium ~ stay well clear of them and use modern technology lubricants which will extend the life of your XK engine substantially. Same goes for gear oils.