Two Oils, one's noisy, one not

Slightly confusing title but this is really for info only.

My son has a tuned S13, we have just rebuilt the engine and on Sunday replaced the turbo. It had done about 200 miles on a decent 10w40 with no probs. Fitted the turbo and took the opp to change the oil, but we had no oil, duurhh.

It was Sunday eve, so he went to the local gararge and bought some Semi 10w40, Esso. Tappets rattled on tickover.

Changed it for Valvoline 15w40, quite as a mouse.

Ok, I know the grades are different on the "third" oil, but the first and second were both 10w40. One was quiet, one was noisy.

As has been hinted/mentioned on here before, choosing the correct "make" is just as important as the correct grade and quality.