I have a 1984 chevy caprice?

and i had an oil leak. took it to the mechanic he said it was the front seals. i assume he fixed the leak...when i went to pick up my car he wasn't there to answer my questions and his "assistant" could only show me a bottle of the type oil he put in. he put SAE 20W 50 oil in the car. my ? is do u guys think this oil is ok for this type of car? why would he put such a thick oil in it? thx guys