With the M32 box, we tend to recommend the Fuchs Sintofluid 75w-80 and it's been shown to work very well. WG Motorworks mentioned above do know the M32 very well and they use a lot of the Sintofluid, I speak to Warren there on a regular basis and he has said that he finds it does the job very well (a lot of the VXRs are heavily modified, so the oil is having a harder time than in a standard car). I assume it's still correct, but I don't think WG have had one come back to them since they have been using the Sintofluid (that might not sound that impressive, but with the number of M32 problems, that's actually pretty good). He does recommend regular changes of the oil, but it's not expensive, £30 a year isn't too bad for a change.
The Sintofluid seems to work so well as it's very good quality and a little thinner than the 75w-85 specified by many companies that use the M32. As it's a bit thinner, it gets around the gearbox better than the standard stuff or a 75w-90, but as it's such good stuff, there is no loss of protection.
ATF is often used in manuals (lots of Fords, BMWs, some Nissans and Toyotas, plus others I can't remember right now), without issues at all. There are times when an ATF can be a better option than something like a 75w-80, but some people use an ATF in boxes that require a 75w-90 and it's not always a good idea, especially in high demand applications.