Lots of people recommend when putting a new capsule in you should put a drop of silicone air gun oil on the tip to help it to seal. Well I have a Walther RAM P99 and from the beginning I had problems with the gas leaking in the end I had to return it to Brocks to be repaired. When I got it back (no charge by the way) after a couple of capsules it was doing the same again. In the end I decided to try it without the oil I cleaned it out first and guess what no leaking at all and I have not had any problems with it. The morel to this is perhaps a drop of oil will help every now and then but not to often.
I should add that the oil in question was the stuff that came with the gun from Umarex not just any old oil.