Oil temperature vs. viscosity

I've isolated that it wasn't the 10w60 Castrol TWS being bypassed by the pressure relief and not being cooled. I took the 912 for an extended drive across the border into Indiana about 20 mi each way and actually had to turn back to Indiana since there was a slow freight train blocking the route. The oil took longer to warm up, but this time I traveled with my calibrated oil temp dipstick and recorded 240F. The weather conditions were 85F, 71% humidity, and 29.98in of Hg. I noticed that very quickly, the oil would drop some 5-10F just by pulling over to check the temperature. I also tried keeping it out of 5th to keep the rpms up and that was good for a few degrees, about 5F cooler at the max. Either way, it's hotter than with the Royal Purple.