Well Anne I always fill mine with Shell Rotella 15/40 same as the motor oil. I fill mine as the manual calls for it, and on mine it says on LEVEL ground, or cement

, fill the oil to the plug level, in other words when you remove the plug, the oil should be a the level of the plug, some might just leak out.
Owned and drove a 2000 Freightliner FLD 120, for over 1,200,000 miles never had a problem with the front axles using that criteria.