When a multi grade oil gets
severely overheated, the oil can thicken up depending on its additive
components.

that is also true of single weight oils, but you knew that didn't you (you
being a service manager and all)?

petro lub oils usually get pretty damned gooey at about 300F, hence one of the
advantages of syn oils, which make it usually to about 500F.

petro oil can cause exhaust valve sticking in high performance engine of long
ago designed to run on leaded fuel (which lubed the valve stems and seats) that
now try to run unleaded.