This is an area of rather heated debate -- some people advocate silicone (DOT 5), others recommend glycol-based (DOT 3, 4, and 5.1). Glycol and silicone are not miscible (can't be mixed), so do not use one if the other is already present. Original equipment would have been gycol-based fluid, so the odds are enormous that that's what you've got in there now. DOT 3, 4, and 5.1 are completely miscible with each other, but I would not recommend you use anything less than DOT 4. The most recommended DOT 4 is Castrol LMA. I think the DOT 5.1 is Valvoline Syntech or something like that, but I've never used it. It's supposed to have most of the benefits of silicone without the drawback of having to completely flush the system of all the old fluid first. Unlike silicone (and like all glycol-based fluids) it will damage paint, so clean up spills immediately.
Taken from a MGB site. :wink: