1) Air cooled motorcycle oils , air cooled mower oils and air cooled car oils all have different addative packs and are nothing like the same.
2) 20W 50 was the most common car oil till around 2000 when to meet with the EPA regulations on start up thinner oils were used which allowed the engine to spin faster at start up.
3) thinner oil in the hydros will leak a bit faster, but lubricate better then the heavier oils.
4) the thinner the oil the better it conducts heat so thinner oils heat up faster and cool better than heavier oils.