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Q: Oil leak from gearbox of Cevy Express AWD on 2004 Chevrolet Express 2500

I have 2004 Chevy Express AWD. There is app. 80 000 miles driven with this car.
Recently local service center did full oil change (axles, gearbox, intermediate box and engine), as well they change all filters (engine and gearbox).
After i start driving, on speed app. 80 mph gearbox spill (splash) out oil. It was so much at once that there was a smoke behind the car (exhaust system was hot). I stopped the car and there was leaking maybe 100 mg of oil to the ground. Checking oil level it shows that there was a little below minimum mark. Then i was continuing driving with 55 - 60 mph and there was no leak anymore and oil level was keeping the same a little below minimum mark. If i try to add oil, all it leaks out until this minimum mark. But leak was only during driving, not if car was standing.
Service center try then to find what is wrong but they failed. They estimation that maybe oil was leaking from overflow-overpressure pipes.
Gearbox was filled with oil up to maximum mark and i start driving. With speed 55-60 mph it is very OK. No leak.
If the speed rises more then 70 mph, after some 10 minutes of driving if splash the oil out again. And again if driving slow speed, all rest of oil until a little below minimum mark leaks out. Service center rise hands and say that they do not know what is wrong.
Today some private gay take the gearbox away from car to check oil seal etc. in turbine area between engine and gearbox.
This area is dry, so no oil leak from oil sealing there.
All diagnostic system shows that everything is OK, as well gearbox is still working like a new one, no problems with change of gears, no strange noises etc.
Please can somebody advice what can be a wrong if gearbox is doing like that and how to fix this problem.
CHEVY owner from Europe
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