There were two types of M47, I and II. The early one, used up to '86 on 740's and never used on 240's or not at all in the US, has a cast iron case and a speedo output gear with a weak 5th gear intermediate shaft and gear set. This was because it was the largest gear set that Volvo could fit in the transfer case because the speedo gear was required. When they added the rear differential inductive electronic speedo pick-up, the 5th gear and transfer case of the type II used on 240's from 87-93, was re-designed with a more direct drive 5th gear that can handle the power, along with more torque transfer to the rear axle. It is as strong or stronger design than the M46 OD unit, and should last forever, even if you keep the tranny sealed with the original Volvo synthetic fluid.
I know, because I just changed mine at 270k miles for the first time. It is in perfect condition, no wear in the syncros, bearings or gears, no metal particles on the magnetic plug. The oil that came out was clean and clear.
I did put the Redline MTL in there, but I really considered the Volvo special order synthetic oil at $28/qt. After having done the change, I figured I just wasted an afternoon and $20 on the Redline.
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'89 245 Sportwagon, '04 V70 2.5T Sportwagon