Fermin,
I had the same problem on mine. Every time I adjusted the valves I seemed to get a leak, first one side and then the other. I bought a set of silicone valve cover gaskets from an independent BMW shop in Deland FL. http://rennsportmotorrad.com/ They weren't cheap, about $35 for the set but they never need to be replaced. Just wipe them clean and use again. No leaks at all and I have had them on/off a half dozen times. Mine are red because I was in a anxious and that was all he had at the time but he has them in black and I think grey also. Well worth the money.