i dont have any personal experience, but this post on bobtheoilguy is consistent with your experience.
'Heavy oils can for sure cause hotter running temps.
My empirical and vicarious experiences show thicker oils have more drag and cling, and can cause hotter oil!
Something like temps 10 deg F cooler on the highway with 10-40 vs.20-50. '
Higher viscosity in summer heat?? - Bob Is The Oil Guy
posts 2259028 and 2260060
you switched from 10-30 to 20-50 with a 25*F increase, and the other post switched from 10-40 to 20-50 with a 10*F increase. roughly, each 10 point increase in oil weight leads to a temperature increase of 10*F.
admittedly a limited sample size...