Luckily this happen on the last run in and did not loose much oil. When i i arrived at the landing i noticed a film behind the engine. Pulled the cowl and the oil cap had popped loose. Not sure why this happened. Tried to tighten only to discover it will not not tighten up. Havent got to fully investigate to decide if its the tank or just the oil cap. So looking on Europeans site at the tank it appears the cap is sold separately or does the tank come ready to instal? Could someone verfiy the part number of the TANK ASSEMBLY needed. Why would this cap just come loose?