75W140 has double the viscosity at 100 C than 75W90. Using it will consume more power. It provides better gear wear protection , but LT230s do not wear their gears, so it is a waste of time.
The input spline wear problem has nothing to do with the oil. The problem is within earlier boxes as there is no way for oil to reach the splines. The "fixes" provide means for oil to reach the splines. Some people believe that a lot of the wear is due to misalignment of the shafts.