How long should it take to change a tappet cover gasket? Cummins says that is where my oil leak is at.

Oil has no factor in the one I did it had over 620K miles and its a 1992 Dodge Cummins 12V it was pure old age and the gasket srunk and split open like KOYOTE mention... Sorry to say nothing last for ever...

both had extremly hard and split gaskets long term heat exposure i guess, dont think oil type is a factor at all but i do recommend rotella with lucas additive ive rebuilt these motors with major milage and this oil diet and still seen factory crosshatch in the cyl. bores so im sold

Heck... My 1983 Goldwing GL1100 had crosshatch still at 85K miles when I blew the head gasket and that just standard oil engine oil 10w-30 or 10-40w and honda filters... All I'm pointing out is that today's engine oil is head and shoulders above yester-year... Just something to consider because there is at least 2 members here with over 500K miles on just standard petroluem oil...