Reply to Cat Yellow1:
Thanks everyone. That's what I was hoping to hear.
I have two D2 engines. The 5J runs in a small bulldozer and I have never been able to run it hard enough to get the oil anything near "hot". It runs the stock il cooler and still doesn't leak.
The other is a 5U engine converted to a generator engine. Once started and warmed up, it puts out about 18 kW steady--no letup. I have fashioned a home-made oil cooler and it helps, but the oil gets pretty hot. I can still put a hand on the filter housing and not get burned, but you wouldn't want to hold it there.
I think the question involves the service to which the engine is applied. On a generator, a pump, marine power...get an oil cooler. On a tractor constantly moving toward cool air and pulling variable loads, maybe not. But, I live in the Northern Oregon Cascade Mountain area. I have never run either in 100+F heat!