My experience with my 03 with 150k miles is: use 80-90 weight oil and fill to the bottom of the red cap opening. Later models have a fill plug or perhaps a fill level line on the stemco hub but the 03s didnt. Note, the wheel hub itself has a fairly large hollow for oil capacity so whether you fill it to the red cap opening or find some way to get more in probably doesnt matter that much. Whether you use synthetic or dino oil is a personnal choice. Assuming your have dino now and want to switch, the only way I know of to completely drain the oil out of the hub is to remove the hub--not a project for the "faint of heart".....
[might get by just removing the outside bearing, but]
2003 40' MDTS
Garden Ridge, Texas