The owners manual of my 1973 Volvo says to use either 30W or 20W-40W oil in the M41 4-speed plus overdrive transmission. In the late 70s, Volvo did something to the overdrive unit and then recommended Type F ATF. A lot of Volvo owners use Red Line MTL in the M41s and the later M46s. I don't get it. The overdrive unit bolts on to the back of the 4-speed transmission and I guess acts like an extra transmission. It is activated by an electric selenoid. Is there a significant difference in the viscosities of 30W, 20W-40W, ATF, and Red Line MTL? I'm not worried about the 4-speed box. You could probably put mud in it with no problem. But the overdrive is much more complicated.
Thanks
Jim

quote:

Is there a significant difference in the viscosities of 30W, 20W-40W, ATF?

Trans fluid is thinner, and is meant to provide the right friction for the clutches and bands in transmissions. As many motorcycle owners have learned, some engine oils are not the best for multiple disk clutches.

bill h is correct; ATF is around SSU 27 (~SAE 10W). More important are the additives that provide slip-stick differential, as clutches are designed to slide into engagement (GM) or lock up firmly (Ford, Chrysler). However, since the OD is self-contained, motor oil would do. I'd follow Volvo's book recommendation, but also check with the local dealer for any TSBs.

Jim, This transmission is a Volvo four speed with a laycock overdrive unit mounted on the back. Ironically this is the same overdrive unit that Gear Vender's uses for the overdrive on the TH400 on your Barth. The transmission and the overdrive share the same fluid.
Volvo had a TSB that called for type F fluid in that transmission instead of 10W30 motor oil. The reason I was told was for smother shifts in to overdrive. I have used Dextron III (like the TH400) and it worked fine. Redline MTL has the same viscosity at 10W30 oil so should be fine in your climate. http://www.redlineoil.com/product.aspx?pid=45&pcid=7

I have used Redline MTL in many other European cars In my shop and it makes for smother shifts. I have never had the opportunity to try it in an old Volvo OD though. I would give it a shot and see how it shifts.

Be VERY careful with you fluid selection, I had many Triumph cars (sympathy please! :+)) with the same OD unit and I learned NOT to use any oil other then the oil recommended. Synthetic oil ruined my first O/D unit because it is too slippery, got into the friction material and the unit would not drive. The unit is a planetary drive system with a conical friction driver, unless they have changed that material it does not like supper slippery and penetrating oils!!

EDIT:

I just called Gear Venders and they now recommended a synthetic oil:

Castrol SYN TORQ 75/90 in their O/D boxes,

The previous "J" model box does in fact use an asbestos material that required a mineral oil with NO additives. The later models including the one they sell now has a "Raybestos" clutch which is synthetic oil tolerance.

If you get the serial number and call Gear Venders, they can tell what the proper fluid should be for the application.