Changing oil viscosity won't help. Here's an idea that might. Domestic engines ran 11/32 valve stems in lots of versions. The Chrysler slant six comes to mind but you'd have to check. Their seals were umbrella type and sat on the stem above the guide. The problem is the Volvo spring inner diameter might be too small to accept them, but a bit of trimming might be possible. Then you merely have to remove the springs with compressed air in the cylinder through a fitting in the plug hole, and remove the retainer and spring and slide them on.
Otherwise pulling the head and having the top of the guides machined for a domestic engine 11/32 positive seal like the B20 one only bigger ID would be a solution.
That's assuming the guide to stem clearance isn't too large to begin with. Normally B18's hardly wear the guides.