why not switch to synthetic after 40k?
I think u have a reason.
True. There is high probability of leakage from gaskets that have been plugged by “sludge.” Synthetics including the detergents tend to dissolve them and people have reported engine oil leaks after the change.
I am told that the synthetics swell up the seals and gaskets so if you ever go back to the other you get leaks all over the place.