As Kitty says, use GL-5 in the diff. In a lightly-loaded rear end like the non-Turbo 240's, there's not much point in using a synthetic - especially if you don't see very cold temperatures. I'd say just shop for conventional 75W90 in a name brand like Castrol, Shell, Pennzoil, etc, in a GL-5 rating. That would probably be as good or better than the lube Volvo put in at the factory.

Make sure you can remove the filler plug before draining the diff! Some unhappy stories here in the past from folks who didn't.
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Son's XC70, my 83 244DL, 89 745 (Chev LT-1 V8), and XC60. Also '77 MGB and four old motorcycles. Long gone: 1981 244, 1994 940 and 1998 S90.