Oil pressure & weather conditions

[3406E]

Cm870- with the hot weather my oil psi went down to 28-29 today.It usually runs at 31-33 @1400-1500 rpm going down the road.Had to pull in a truck stop and cool off also added a quart of gear oil. It went back to normal but after about 3 hours of driving I was back to 29.The humidity of the weather went up to almost 100 degrees today. Sadly I wasnt paying attention to my oil temp .I’m just at 8k on this oil. Rotella T6 full syn. Anytime it does this it does not change the idle psi which is at 20-21 always. This is not the first time this happened last year summer in Miami and Houston which as you know are very hot place. I usually just change the oil when this happens but I don’t want to keep doing it wasting money. I change my oil every 12k.Im almost at a million millions miles ; should be getting it rebuilt in no longer than a month or two the most. I also planned to get a new oil pump when rebuilding.My buddy runs a couple cm871 in dump trucks and he said he seen his go down 20-25 lol when it’s really hot,his exact Words were “it’s not a cat ”and he runs them like that. I wouldn’t dear try with mine though. What are your thoughts with hot weather and oil psi?have anyone experienced this? How much of a drop in pressure is aceptable or is it aceptable at all?