DeCaff2007
I'm Awesome
Would not hurt a bit! The newer 4L60Es are built a little better than the early ones, but any transmission is happier with cooler fluid. And if you tow, DON'T tow in overdrive (put selector in D instead of OD).
That's some sound advice right there.
If you are worried about the mileage on the L31 - don't be. Vortec engines run forever when maintained. As for the 4L60E, if it's not giving you problems, then don't look for any. Again - proper maintenance.
It seems you've run across a decent Burb. If it were me, I'd pull the trigger on that deal.
EDIT: BTW, the OP has better spelling and grammar in English than most of the idiots around here (NEPA) and he's in POLAND ffs! Good job @SpitShine_PL