How did you work out it was harder to snap the hose with water than with oil? It'd make no difference at all. If anything it would be harder to snap with oil in because it's slightly thicker, so it would be slightly harder to flex the pipe. Ever pulled the brake when it's empty and theres only air in there? It's fairly light I seem to remember... Oil feels less responsive and heavier to pull than water because its a thicker, more viscous fluid. I use a syringe and I never get air in there, it's all about the technique you use. Bleed from the bottom and have the run off pipe in a tub of the fluid, so when you remove the syringe from the slave it keeps sucking fluid in instead of air as it runs down through the brake. Makes a mess, but it stops it sucking air in.