If you have two motor oils:

Mobil 1 15W50 and another Oil SAE50

At operating temperature which oil protects better? Are they about the same
thickness?

Now what about at a low temp below freezing?

I have a slight knock in my engine when it heats up and the dealers service manager says I should just use 50 weight oil. Does this make sense. Comments would be appreciated. He says that it would be too time consuming to find the problem at this stage. Would have to use plastigages? all over the place because the amount of play would probably be very small.

Thanks,
Michael