Just to stir the pot, 90 wt gear oil has essentially the same viscosity as 30 wt motor oil. 20w-50 flows like 20 wt when cold and 50 wt when warm. 30 wt does whatever it does at whatever temp,i.e. less dense when cold and "beaks down" more when hot than 20w 50. I stick by the idea or what does your manual say? Bob Johnson BJ8