The car to which I was referring, a 1985 Volvo 245 Turbo (manual, by the way, great mountain car) had an oil to air cooler with a thermostat. So, clearly, the thermostat opened at 85C. Which was what the service manual stated. And clearly, there was enough cooling capacity to handle easy cruise on a hot day.
I think that there must be performance differences in how the oil cooler is sized, and plumbed. More area vs. less internal to the cooler. Oil to air vs. oil to coolant. I’m surprised there was any variation in yours, I would have thought that under low load, the oil would stabilize at coolant temperature.
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I don’t have a way to measure oil temp on this car, but I just upgraded the oil cooler on my daughter’s XC90. It’s an oil/coolant design. Much thicker ”sandwich“ on the new one (right) means more heat exchanger area. A bolt on upgrade, using a diesel part in place of the original part, so, all factory fittings.
I would love know if it makes any difference. Clearly, Volvo thought the diesel needed better oil cooling, but since I wasn’t able to measure “before” I would have no data to know if this made a difference. My goal in upgrading is to ensure consistent oil temp over a wide variety of operating conditions.