I alternate between 20W50 and, I think, 10W40, depending whether I do the oil change or a JLR garage. The latter use 10W40, so far no noticable difference. Anyway, I change every 10k km or once per year, old engine designs and a 42 year old engine and all that. The big benefit of 20W50 is that the LT95 gearbox and transfer case take the same, that makes maintenance easier.
Those original cams do live quite a long time so, even if worn out. Since some aftermarket ones are not that much more expenssive than the 3.8l ones, that's what I had installed when it was time to replace the old one. Engine got some more low rev power as well.