The problem with grease or thick oil is the possibility of cavitation starving the gear-teeth whilst under load, so don’t use anything heavier than a straight SAE 90 grade (although modern oils nowadays don’t usually have the sulphorous addatives of yesteryear which divoured bronze – but best check with your supplier.
Using thicker lubricant is only attempting to mask the design fault of poor or non-existant oil sealing – which can be easily overcome by turning out the kick-starter boss and fitting a lip-seal, then using a modern synthetic gear oil.
