Transmission shifting issues

[abates362]

Having some weird symptoms and can’t pin point.

Flat ground, very slight head wind, highway speed. Cruse control on. Transmission temp not quite operating temp. RPM’s stay steady, until downshift to 7th. The truck will shift to 8th and my L/100km will slowly creep up from 15 to 30L/100kms then down shift to 7th again.
It will do this until the trans gets to about 50-60C and then runs like normal and will hold 8th gear all day even on slight inclines.
It will do this in tow haul, does it in eco mode.
Has anyone else had weird symptoms like this?

 

[PaleFlyer]

Talked to a buddy a while back, and he said that modern vehicles try VERY hard to hit some preferred temp, and keep it there. So it probably is shifting back and forth to hit the "programmed" temp, as thicker fluid is more viscous (Tranny fluid is like water, so the logic is stupid in this case, but like Gear Oil this would make some sense.) So it's probably "correct", although its just another case of "stupid modern design" trying to save 0.01L of gas but actually using more, and putting more wear on the system. I can't verify this myself, as it's 15 minutes from the house to the interstate, and unless it's like 3am, it's always stop and go/traffic lights, so by the time I get somewhere I could test this myself, the truck is warm anyway. (Leaving work is about the same, but always in rush hour, so even ON the interstate, it's stop and go for 10 miles as I work on the edge of a major city.)