All you may have to do is turn the engine off and knock the cat converter with a stick or something and see if it rattles. It will be a higher pitched rattle than the rod knocking. The noise should be easlily separated from each other with the hood open.
As for oils. A lot has changed since these engines were first engineered. Synthetic oils, more additives, oil for older engines and so on. As an engine gets older the clearances increase and thus a drop in oil pressure. Most people go with age and the temps from where they live as to viscosity choice. I live where it gets very hot in the summer, but not too cold in the winter, so I use a 40wt during the summer. If I had and engine that was rattling and was trying to make it last a little longer, I would use 50wt, STP oil and a can of engine restore for 6 cylinder cars. You can't expect that engine to last too long and the longer you do run it the more expensive it will be to rebuild.
Don't break it. Get it diagnosed by someone who can pinpoint that noise. Let us know.
Russ
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