Slasher oil seals are notoriously leaky, because stuff often wraps around them and damages them. A light grade grease is usully recommended. Maybe NLGI grade 00 (or 000 or 0) instead of the usual grade 2 general purpose grease.
Roger
This is the flail mower, so nothing ever goes near its seals. I'm sure they are still weepy, but not enough that I've needed to top up the oil like I used too. The slasher here seems to have grease in its gearbox, so I've always just left it alone
The post hole digger leaks quite a bit. But I just top it up whenever I use it. Its used so rarely its better to just throw a few hundred ml of oil in it rather than pull it to bits (and end up in a situation just like the tractors hydraulic filter no doubt) again.