As with any uncontrollable heat source, it is absolutely essential to have an open system. Extend a pipe (preferably with a 'U' bend in it to help stop heat loss) from the hot side of the panel vertically such that the level of the fluid in the system does not flow out the top. This will ensure that the system is always near atmospheric pressure. If the circulation of the heat transfer fluid stops for what ever reason, the stagnation temp of you panel may well be high enough to boil it (at the very least cause it to expand somewhat). If this happens in an open system then all that happens is fluid bubbles out the vent. In a closed system you could potentially have a catistrophic failure! I don't know that vegetable oil is the right transfer fluid. I'm picking that it would be rather viscous at lower temps and this might complicate pumping. I'm sure it would also have a limited life. Pipe sizing depends on a number of factors. I use water in my system and have 3/4" copper throughout. The only time you can use a thermosyphon (convection) to circulate fluid is when the heat source (panel) is lower than the heat sink (radiator). The heated fluid rises to from the source, is cooled and falls back to be heated again. A standard type heating radiator such as may be used with a boiler will not be particularly effective in this type of system. The surface area of the radiator is too small due to it lack of surface area. Panel efficiency goes down when the temperature goes up. The higher the temp of the panel relative to ambient, the more heat is lost back into the atmoshpere. If the ambient temp were say 25degC the panel would typically be 80-90% efficient if you were drawing off it so that the temp was 26deg (fast flow) and be less thatn 40% if you were drawing off it such that the temp was 60deg (slow flow). This makes the solar pannel inefficient at the temperatures that the radiator is designed to work at. My system used the whole floor of my house as the radiator, so it can transfer a lot of heat with only a small temp difference (fluid is at 30degC)