Most Volvo engines, including yours, benefit from a long drain interval oil - an ACEA A3/B4 for example. The Achilles Heel of Volvo inclines of your vintage is the "Flame Trap" or PCV system. It's complex, with lots of oil passages, some of which are heated to help prevent the accumulation of sludge.
But despite the heating, sludge still happens. Your best defense is either a long drain (stronger TBN) oil or to shorten the OCI.... if the system sludges up, you'll start getting leaks. Left unchecked, with excess crankcase pressure due to a plugged up,PCV system, many engines will blow a camshaft seal, requiring a new timing belt, and seals (about $600) in addition to the replacement of all the PCV components (another $600+).
Ask me how I know...