This is what I did last night when I had to get my old tire and split rim apart: carefully stick a screwdriver (I actually used an old chisel) between the tire and rim, try to pry the tire away from the lip of the rim, repeat every 2-3 inches around the rim, eventually the rim half will slip right out.
I wouldn't use oil as it may damage the rubber, instead I'd use liquid soap or silicone spray if you really think you need a lubricant.