Hi BJ,
I believe that the #1 hydraulic system is cooled in the #1 main tank, and with the higher load, will experience the highest heat transfer.
As a sidenote, our B737NGs have fuel tank temperature sensing for all 3 wing tanks, #1, CTR, and #2.
I only mention that only to show that there is a very big variation in 737NGs out there!
Cheers...FD...