My S90 manual calls for 10,000mi oil and filter change intervals if the car sees mostly highway driving and avoids trailer towing, extended idling, mostly short trips, etc. If it sees those latter conditions routinely, then 5000mi. (It holds nearly 6 litres/qts, compared to most cars' 4 litres.)

This contemplates conventional oils, as the book only mentions synthetic oil as the better choice instead of 15W40 for extreme loads or very high outside temperatures.

I use Mobil-1 5W30 year round and with ~90% highway use, I change it at the 10,000 mi interval, always with a Mann W917 filter. Mobil-1 "Euro Formula" 0W40 would be another option.

The timing belt change interval is 70k mi. in the manual, and on an official-looking Volvo sticker on the belt cover. Many recommend installing a new water pump at every second belt change, as it is driven by the belt and a pump seizure would probably break the belt.

The automatic trans is an AW 30/40 and was also used in the Lexus 400 of the day, a 4-litre, powerful V8. A fluid change every 50k mi or so wouldn't hurt, but the owner's manual does not even show how to check the level, much less specify a change interval.

If you still have the car's original radiator, it is on borrowed time.
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Bob: son's XC70, dtr's '94-940, my 81GL, 83-DL, 89-745(V8) and 98-S90. Also 77-MGB and some old motorcycles.