Center section getting tighter

i have a mark williams 9.5 in rear center section, set up by Mark Williams , has approx 30 runs on it. Looks fine, but it is getting tighter than when I first put it in. Used the heavy gear oil they recommended. Why would it get tighter ?

 

It's laying over --bending the gear

 

Well I removed it today and looked it over real well. Found nothing wrong. Set it in front of a big diesel heater until it heat stoked to approx 80 deg F. Spun a lot easier, put it back in car and switched from 75/140 gear oil ( Valvoline ) to a synthetic Richmond 75/140 now it's a lot better. Guess the old heavy oil and 40 deg parts made it tight.

 

I have a toms 10in gear in a strange ultra case. Set the lash where he said to and run ht redline oil. Still moved around and tightened up. I strayed to notice it when I get pulled down the return road. Before that I ran us strange and would only get 30 runs out of it before it would stretch the pinion out of it. I would love to know why I keep having this problem.