My new to me 1984 Volvo DL wagon with the M46 manual 4 speed overdrive transmission slips in and out of OD when the engine is hot. So after about 20 minutes of driving, I can engange the OD for about 10 minutes before it slips out. In reading the 700/900 FAQ on the subject, I went out to buy synthetic ATF type F and couldn't find any. Mail order the stuff is real expensive too.

I called up Steve (the old man on Volvos) at Carlsen Volvo in Palo Alto, and we talked (actually he talked and I listend) for about 5 minutes about my problem. His quite and temporary solution is to drain out the fluid and clean the OD pan and fill it up with SAE 20W50 motor oil. That's what I wanted to hear. He stressed that this is a temporary solution until I can save up the money to rebuild the tranny. I have no real desire to rebuild the tranny.

But he said that it would be fine to use 20W50 motor oil. I did and it works fine. The OD doesn't slip out when hot. I drive it for 15 miles with no problem. I'm fairly happy.

Since you and the car live in a mild climate, the thick lubricant in the trans is OK as a temporary solution. The OD is experiencing intermittent engagement when hot because the fluid thins out enough to get past the leaking piston seals faster than the pump can keep up. Pressure drops, OD clutch fails to hold engagement. As you know, the real solution is not thick fluid (may work OK for a while, tho), it's to pull the OD and put a seal kit into it. No rebuild of the "transmission" needed.

"OD Guru" Duane Hoberg sells the parts and all the advice you'll need for this. See also the 700/900 FAQ's for more info: http://www.brickboard.com/FAQ/700-900/OverdriveRebuild.htm

After the rebuild, fill the trans/OD with Redline's MTL. This alone has largely cured my hot-slipping, shuddering OD for 6 yrs plus now, as I await the time and ambition to do the right thing. (Yeah, I know I'm not practicing what I preach here...)
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Bob (son's 81-244GL B21F, dtr's 83-244DL B23F, 'my' 94-944 B230FD; plus grocery-getter Dodge minivan, hobbycar 77 MGB, and a few old motorcycles)