As far as I understand it, the first number relates to the cold viscosity and the second relates to its maximum usable viscosity.

In other words, a '0' or '5' (first number) graded oil will work well from cold, but offer little protection to a worn engine.

A '50' or '60' (second number) grade oil will be effective at higher temps, so for use in a powerful car.

To quieten that tappet's down, I would suggest a 5w40 or a 10w40.

Not saying I am definately right though.