1800ES - M41 Manual Transmission and Overdrive Oil Type Topic is solved

[30volvos]

Hello,
My mechanic drained my M41 transmission/Overdrive on a 73 1800 ES and replaced it with Automatic Trans Fluid (ATFATF1).
He said it was filled with the wrong type and said the ATF is standard now
The manual recommends using SAE 30 or Multi-grade SAE 20 W-40.
I am hesitant to drive the car until I determine which is correct.
Can anyone help fill me if the requirement changed?
Thanks!

[jimmy57]

For years atf Type F was used but overdrive problems and manual trans bearing issues were much higher in US than in Europe where motor oil or a special synthetic oil was specified ( Synthetic 10w30 is all I ever use and it has fixed many Laycock overdrive shift delay issues). Redline MTL is also fine. They specifically advised against anything but Type F if ATF was used as friction modifiers for smooth shifts in Dexron ATF fluids will make the cone clutch slip in reverse on models with overdrive unit. There is a freewheel in the OD unit that drives you forward in when OD is not on. That freewheel (one way roller clutch) is turned in its non connecting direction in reverse and the return springs in overdrive push it onto a beveled edge of the output shaft annulus gear. If the fluid has the firction additive that lets auto trans clutches slip a bit for smoother shift then it will let the cone clutch slip when you try to back up a hill or get hard on throttle in reverse.

[30volvos]

Thanks Jimmy for the background and details!