80/90 dino or 75/140 synth

So my buddy and I went out to his shop to change the diff fluid. I bought everything ahead of time according to Haynes manual and owner's manual.

Anyway, AFTER we drained the diff fluid I noticed that a tag on the cover of my 2001 GT diff said use 75/140 synth only. Of course we had 80/90 dino oil

Car was apart, no choice but to use what we had... That was 3000 miles ago and so far no problems. How high of a priority should I make it to correct this situation. Do I even need to? And yes, I did at least have the friction modifier and used it to spec...

ttt

I am going to switch it back when I install my gears in July. I was really just trying to decide if I could wait that long.

Found out my buddy who is a highly skilled home mechanic now has a magnetic base dial gauge and an inch pound torque wrench so it should be possible to do it myself with his help and tools.