Yes they do, and it is probably what most people should use.
For me because I'm willing to put up with the oil warm up period I use other oil. Apparently customers weren't, according to an old dealer service guy I used to know in Canberra, and volvo use an ATF type oil for "get in and drive application". It makes sense given the overdrive internals.
However on the big6 Healey, Triumph and other machines with not dissimilar mechanical and somewhat truck like gearbox's and the exact same overdrive unit they use a more conventional oil with better performance for the gearbox side of the equation (bronze parts, tapered roller bearings, needle bearings. I've taken this avenue as I don't usually take an old car to the shops for five minutes.
Interestingly IPD make reference to the use of engine oil in the M46: https://www.ipdusa.com/Articles/597/M46-M47-Transmission-Oil The thing with engine oil, especially modern oil, is they are short lived and have additive packs with chemistry for mopping up combustion produced. I was looking for something with the additive designed for wear/lubrication under tougher conditions (like a good drive on a bendy Tasmanian road). ATF is a bit water like when it heats up for my liking, given the very conventional nature of the gears inside the box.
Supposedly this is the oil Volvo used to recommend prior to the change to ATF, much more of a conventional gearbox oil. I think the change occurred around 1984?
So back to the original problem, a good few drives with the ATF to see what happens, then fresh ATF (if its a second hand box, good to do anyway) might fix the problem? Another oil down the track might be another option to look into? I know I have had both slipping and non-slipping outcomes depending on the choice of oil.
This assumes its not clutch related, or the gears are not slipping out at the selector momentarily under load, a bit like the earlier box's when the brass surface becomes a little worn.
