volvo says 30W because of slippage of O.D. friction material. irv gordon went 2.6 million miles before trans was taken apart, 3rd gear syncro went hooey. im am .00001% chemist, isnt shear on tooth contact 90W is used.i work on cars,im not a writer.any thoughts. working on 1800s for driving once more. B20 M-41 and 4.10 rear gear.in garage for 25 years,time to get rolling again. i am third owner. have original papers from new at dealer. 1965 is black. what about available black in 1965. selling miata?!? to throw money at 1800. just brakes and tires. i can twirl the wrenches.keeping stock height and tire size so i can get out of the car. parting out #1449. Al

thanks for the input.

Although Irv runs 90w, I have never done so in 30+ years I've been repairing Volvos for a living.

30w is what Volvo recommends and that is what I use.

It might be working for Irv for 2 reasons. 1st his M41 has a "D" overdrive and possibly those will tolerate thicker oil. 2nd, IF he is using GL1 like I get from NAPA, its mineral oil based and visually isn't as thick as the GL5 I use for most rear ends.

If you have a later M41 with a "J" overdrive, I strongly recommend against 90w.

FYI, Volvo recommends ATF for the M46 that also has a "J" overdrive and they are directly interchangeable from 1976-1985 Volvos with a "J".
--
Eric
Hi Performance Automotive Service (formerly OVO or Old Volvos Only)
Torrance, CA 90502

At Ford’s 50th anniversary celebration in Dearborn back in ‘03, Irv told me the tranny had been gone through once or twice - he was +2 million at that point. ‘Course, 98% of his miles were done in 4th/OD...
--
82 242-6.2L; '17 Mazda3; '16 Crosstrek