For a while now I've noticed that my mechanic shop puts 20W50 oil in my 30 year old Corolla, which is I think a somewhat unusual grade or weight of oil. I remember back in the USA they usually used 10W30 or something like this. I asked the mechanic about it and he said that a 'heavier' grade of oil was appropriate for such an old engine.

Can anyone comment if he's following a good policy here? I do find that there is a very long period of valve clatter (and oil light) when the engine is first started up in the morning.

The oil is probably correct for the age of car. Most engines make a noise until pressurised oil has reached all parts. Don't put a thinner grade oil in.If it runs OK and your oil light stays out when warm and at idle then all is OK.

Yes he is, and quite frankly it's surprising to hear..