Ayuh,..... Donno 'bout Volvo, but Merc drive nuts are Not ssteel everybody thinks they are,.....
They're chrome plated Brass, fine thread,...... Tough to find, other than from Merc,......
Ssteel nuts tend to gall badly if the nut isn't oiled, brass is quite the opposite,.....
I'd Guess Volvo does the same,..... I'd use a washer, without the shouldered nuts, myself,.....