This is my first Tahoe with the manual-shift 4WD option, all of my others were push-button/auto. I've heard a lot of debate about this, but can 4WD be left engaged for extended periods of time? I don't mean hundreds of miles, but can it be used for say 15-20 miles continuously? We live in the northeast and we have had a ton of snow recently. Sometimes my street is completely covered in snow, but other local streets are dry and plowed. Should I be shifting out of 4WD when there is dry pavement and back into 4WD when needed? The truck has 68k miles on it and I am not sure if/when the gear oil has been changed. Can this particular front differential handle the full synthetic 75W-90 or should I just stick with conventional 80W-90?