If you look up the recommended oil in the owners manual, in average climates 10W30 is recommended. For a high mileage engine, consider going up one grade to 10W40, which is what others here suggest and I always prefer. My wise old Volvo-trained indie mechanic was surprised I hadn't done that sooner.

Additionally, if you've got a leaky engine, consider using a "high mileage" oil. These contain seal conditioners that keep the seals puckered and pliable to prevent leaks. If your engine seals are in good shape then seal conditioners can actually make the seals wear faster, so in that case high mileage oil isn't recommended. However, if your seals are weeping then a high mileage oil can buy you more time until the next T-belt change when it's more convenient to replace the front seals. Of more importance to me is keeping the rear main seal from weeping as it's a major deal to replace it, so I will often go with a high mileage oil when I can find it on sale.

Synthetic is of course preferred over dino oil, but these engines are built like tanks and will happily run on dino oil. More important is to change the oil and filter regularly, using a quality oil and quality filter like the Mann filter that many of us religiously use. When initially switching to synthetic, consider an initial early oil change as a lot of old deposits will be loosened. The dipstick can give you a good idea how long the oil is staying nice and clean.

Here's a good article I found on 10W30 oils and high mileage engines
https://www.autozone.com/diy/motor-oil/viscosity-5w20-vs-5w30-vs-10w30
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now