My daily driver junker truck is a 1992 F150 with the 4.9 . . . Operating temp is always just under 210F (read on a mechanical gauge).
To try and quiet a lifter tick, I added a bottle of the Rislone concentrate to the Traveller 15w-40 that's in it, probably at about the middle of an OCI.
Truck was/is immediately running about 5 degrees cooler. Nothing else was changed.
I don't think it was any reduction in viscosity, as it ran the same temp on SN 10w-30.
It is possible, 5 degrees F is quite minimal and it is possible to be impacted by additive added. I have similar observation with Lubegard added to the oil as well. It keeps engine oil temperature colder by 1-2 degree Celcius and the engine takes longer before reaching operating temperature.