988b : Best transmission oil check practice

What is the correct ( or most accurate ) procedure to check the trans oil in an old 988b that has a dripping leak around the rear output shaft on the transfer case while it is working ?

I have an operators manuel that says to start and run the machine at low idle for 5 minutes then check the dipstick.
I usually start the engine and run at a low idle while I grease the non moving grease nipples on the machine, ( around ten minutes) then check the oils. But every time, the trans oil is well...

988b : Best transmission oil check practice