I would buy 3 per axle to be on the safe side but my 12b and d44 only took 2.5 each.
As for weight, depends on your axles and where you live.
In cold weather you need hinner, but if you have a worn axle with big tolerances, if it makes noises, I would go with thicker.
The multi grades are usually synthetic so they are better as well as more $$$