I have never bothered to measure the capacity of the tank I made for my Husky framed 500, but would hazard a guess of about 1.5 to 2 L. I used to run a Lochart oil cooler but found that it kept getting knocked about, so I removed it. Since the oil tank is mounted on the LH side of the bike and fills the area where the std Husky pipe used to go, it gets a lot more air flow than the std XT/TT500 oil tank (frame) I rekon that the tank itself will disipate the heat fairly well as it has a fairly large surface area.
CJ