Had some time to mount the oil temp gauge and test it out. This afternoon it was real hot - around 100 degrees. So I turned off the heater so that there was no flow through the exchanger, and went for a spirited drive down the hill, flogged it up some grades, and the temp rose to what I guesstimate is around 200 degrees, maybe 205. Going over the course again, I turned the heater flow on. Coolant that has now entered the block, flowed through it and exits to feed the heater, then goes last to the exchanger. Nothing really changed, although you can see subtle changes in the oil's temp when the engine's temp goes down a little, and vice versa. In mild weather, my coolant temperature gauge doesn't move up as far as it did today, so I'm guesstimating around 180-185 is the norm. It moves to where it did today when things are real hot, around 100. I'm concluding that the coolant and oil are stabilizing each other and probably within a few degrees of each other, which is per the gauge, around 200 f. Under these conditions, I think the oil is running okay, right, Robert?
What I'm going to do next is to braze a 5/8" tube onto the water pipe so the exchanger gets coolant right out of the pump, which is the coolest source, before going into the head and picking up all that heat, which it is doing now. I think this will noticeably reduce the oil temp.
Edit: since the two conditions are so close, I conclude that my oil temp has been running like this before I put the exchanger on. I don't believe that's a bad thing at all. This also tells me I have "reserve" capacity for pulling out extra heat the turbo would add, or even, say, adding a/c and lugging up some long grade on a really hot day. I'm operating at 2,400-3,000 feet and one stretch is 8% grade for a mile, and I keep it floored in 3rd to hold 50 mph, which means I'm spinning around 3,400 rpm. I still think brazing in a tube to pick off cooler coolant is a smart thing to do.