Castor oil may not be the "holy grail" of lubricants, but you'd be best advised to not run a plain bearing engine like the O.S. Max .65 LA without it. The thick viscocity of the castor oil helps the crank shaft "float" inside of the engine bushing.

TimBle's recommendation of 15cc gasser is an interesting one. The normal drawback of using a gasoline engine on a .60-size plane is the added weight, but as I mentioned previously, this may be a benefit on the Tiger 60 airframe. The nose arrangement on the front of the Tiger's fuselage could make installing most small gas engines problematic, so you should investigate things like mounting width and exhaust options carefully before jumping into a gas engine.

As for the .91 4-stroke option, it's well worth considering if _brad_ wants to try his first 4-stroke engine. The Magnum XL .91 FSR and Thunder Tiger F-91s four strokes are about the same price as the O.S. Max .61 FX he was considering, so it wouldn't even be a more expensive option.

A .91 4-stroke engine typically weighs about the same as a typical .61 2-stroke. Propping the four stroke may be more challenging, as a typical sport prop would be a 12x6 on the .61 2-stroke but a 14x6 on the .91 4-stroke. The tricycle landing gear on the Tiger 60 may make a longer prop impractical due to ground clearance. You can always put bigger wheels on the Tiger .60 and/or build it as a tail dragger. You could also use a 3-blade prop on the 4-stroke to shorten the diameter by an inch (13x6 3 blade is roughly equal to a 14x6 2 blade prop).

There are a lot of terrific choices out there for powering a Tiger 60. As I said before, good luck and good shopping!