Stop Speed shifting. Also you are aware that in the T5 you are supposed to use Automatic Transmission Fluid.
I had an '85 5.0 with the T5 and put over 60,000 on it with no problems when I traded it in and bought a brand new 1990 limited edition SS454 Chevrolet 1/2 ton pick-up.
I was able to get 21mpg around town and 28mpg on the freeway with my Mustang, and with my SS454 I could get 10.2mpg around town and 13.24mpg on the freeway.
Wish I had kept the Mustang. It was a convertible. I did have the SS for 9 years though.
Go to the 'Z' world class heavy duty T-5, but it's not that much tougher. So, either go to a heavier Tremac tranny or go easier on the T-5. I put a 'Z' tranny in my '89 after I ripped-up the factory T-5, & it held up okay up to like 350 hp, but I did kinda soft-pedal it until I got the T-56.