I made the mistake many years ago using "Friction Modified" Castrol GTX 20w-50 in the gearbox and transfer case of my 75 Range Rover, which used also engine oil. The "Friction Modified" might have been a bonus for motor performance and reducing engine wear, but played havoc with the syncro's, making gear changes harder. Syncro's require a certain amount of friction to slow the gear clusters so they mesh smoothly. The oil basicly was too good and gear changes, particularly down shifts became very slow as the oil was stopping the syncro from doing what it was supposed to do. Most modern engine oils have "Friction Modifiers" and other things beneficial for an engine.
The issue here is, that we are talking about gearboxes, which have very different lubricating needs to motors.
If the manual says mono grade oil, like SAE30, I would stick to it, however times have moved on. I use Australian made Penrite Oils for a number of years and just thought I would check what their Catalogue on line says, which proved quite interesting. They have a range of "Classic Oils" specifically for cars where mono-grades were specified. The following is taken from the website
"Penrite Gearbox Oil 30 is a multigrade gearbox oil designed specifically for use in vehicles that originally used the same oil in the gearbox as was recommended and used in the engine. Many vehicles manufactured in the fifties and sixties specified engine oil for use in the gearbox. Whilst this was convenient, a proper gear oil was a better alternative. Gearbox Oil 30 can be used in vintage, veteran and classic gearboxes including those fitted with straight, synchro and epicyclic gears, overdrives and those vehicles fitted with pre-selector gearboxes.
Gearbox Oil 30 is an SAE 20W-60 oil designed for use where SAE 30 oils were originally specified.
•Does not contain friction modifiers.
•Will not cause overdrives and pre-selector bands to slip.
•Does not contain EP additives
•Compatible with synchro cones and other metal parts.
Oils ain't Oils, so engine oil is for the engine, it has stuff in it that will reduce the performance of your gearbox. Not sure if you have Penrite over there, but it would be best for you to check the Oil Companies web sites, to see if they supply oils that are appropriate for the age and design of the gearboxes we use
Cheers Stephen