Try to find out what the previous owner was using. If he was using the recommended straight SAE 30 then going up to a 20/50 will probably produce leaks. However, after an engine rebuild using 20/50 will be OK. The rear, rope, oil seal is notorious for leaking oil and the story goes that they dripped oil at the Abingdon factory floor while awaiting despatch. Definitely stay away from synthetic oil.
Petrol - Use the lowest grade you can get. When these cars came out Britain was still on 80 pool petrol. Higher grades will possibly cause greater heat and the risk of burning a valve or piston. Adding paraffin/kerosene will also help in dropping the octane level. Half a litre to a tank of fuel will make no difference to starting, etc.
I use my MG YB pretty regularly - that's the same XPAG engine as the TD, but single carb, and have had no valve recession problems and we've been totally lead free for nearly ten years.