I have just purchased a 1996 4.5 petrol Amazon and noticed that there was fresh grease around the ball on the passenger side (uk) I wiped it all off and checked the diff oil, which was 1/2 pint low, topped it up, that was 150 miles ago, the grease around the ball has not reappeared, do they somtimes loose grease and do you think I should top up with grease, I assume you take out the bung and screw in a grease nipple and pump up, but how do I know how much to pump in ?