Brake fluid for a series III

Does anyone have any idea what type of brake fluid would be in my series III lightweight? (Assuming the guy we bought it off used the same fluid as would have originally been used)

I heard it can be a bad idea to mix them up so thought I'd best check.
Cheers.

It's quite a late model by the way. 1983 I think.

Dot 4 i think. If its lost some fluid there is a bigger problem and you will probs have to drain the lot anyways.

Dot 3 and 4 are compatible but don't use dot 5.

Dot 5.1 is compatible with Dot 4, it's just Dot 5 thats silicone based, but to be honest you won't have trouble in a series with boiling Dot 4 lol

Ok, thanks for the help everyone. It's starting to look as if brake fluid's the least of my problems though.
Just bought everything to replace the rear brakes, and the new drums won't fit over the new shoes (even with no pressure in the cylinders).
I'm going to put a new thread about it on the series forum so if anyone's got any ideas please let me know.
Cheers.