It is a bit unusual for the valve train to become noisy at 150k, unless something is amiss. With good, solid maintenance, i.e. 4k oil changes, the problem is probably something related to a component failure. Are all the valves noisy?. What is your oil pressure gauge telling you?? Are you absolutely sure it's the valves?? The kind of oil you are using (unless it's junk) shouldn't really affect the function of the lifters. BTW, 150k is not really high mileage for an M103.
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.