After posting I was informed that my lathe has an H taper on the spindle end. It is quite possible that the adaptor sleeve that came with the lathe is 5C inside. So I am am going to find a 5C collet and see if it fits into the sleeve in which case I will buy a set of 5C collets and make a collet closer to fit.
I see you've decided on 5c anyway, but one reason they are preferable over an R8 is that 1" diameter and lower material is able to go all the way through the collet (larger than 1" is also possible if the internal threads are bored out). There is no way to do that with an R8 collet.