I'll weigh in simply because I've had an odd experience. When I purchased my 70E, with an M410, it had a very finicky OD. Sometimes it worked correctly, but more often it refused to immediately engage or disengage. The previous owner's records indicted he'd had it to a competent local Volvo garage and the notations said "internal oil pressure problem." He never got the recommneded rebuild.

I always ran 30W non-detergent, as I had in my previous vintage Volvos. After I removed the unit to replace a leaking gasket, the problem seemed to get worse. I had just about given up and was searchig for a rebuilder $$$$$$, when I decided to try one last thing...80w-90 gear oil. That was about a year ago and the thing has worked flawlessly ever since. I've spoken to that Volvo mechanic and he was surprised. He felt it usually went the other way...switching to 30W from gear oil seemed to help problems. So maybe I'm the contrarian exception.

I'm happy. What I've heard since is 80W-90 has become quite acceptable. And gear oil is certainly better for the tranny. One caution, though, don't use Mobil One or any synthetic or semi-synthetic. I understand a certain amount of friction is required for the OD to operate properly...something to do with internal clutches.