
Also there's the M46 connection. M46 strictly requires the ATF oil because of the planetary overdrive unit (ATF oil without the friction modifiers found in modern Dexron oils, preferrably the nowadays somewhat rare Type-F ATF which is a "high friction" ATF, Dexron causes the planetary overdrive unit to slip and wear excessively). Some Volvo books recommend 10W-40, manual gearbox oil or ATF for other boxes than M46, but many will list the ATF as the preferred oil for all M4x-series boxes.
There's nothing special with these gearboxes or synchros why these wouldn't work with modern fully synthethic gearbox oil. Actually if you would take a 240, a 740 or a 360 with the M45 or M47 gearbox to the gerbox oil change to a local dealer here, it's a pretty good chance you'll get Castrol 75W-80 fully synthethic manual gearbox oil. Currently that's the oil the Bilia uses for normal manual gearboxes (Bilia is a subsidiary company of the Volvo and the largest Volvo dealer at the capital region in Finland). ATF actually causes more wear on to these types of synchors than modern fully synthetic manual gearbox oil, though of course it's not the first thing to be noticed because you could drive these boxes with stock power levels probably forever with the ATF also.

I have that same Castrol oil currently in the box and changes are sharp and smooth even at -25C.

Most important thing though is even to have some kind of oil inside the box, as nowadays I've come across many boxes which have leaking seals and were pretty much dry!

For the diff I've used the GL5 diff oil.
-Marko