Thank you very much for your thoughts on this, it is most valuable to me. I will go for an Oleson screen. It will most definitely be better than the current screen and since this is not my main camera I'll be happy just to be able to use it well enough.
I see that he offers two different focusing aids, a 13mm "micro prism field" and a 6mm diagonal split image "set in a microprism collar".
Would the latter be similar to what I see in my FM3A, i.e. that it gives the benefit both of a split image and a microprism? I mean, I can obviously read that that's what the description says, but what I'm wondering about is the size of the microprism collar. In the FM3A the collar is large enough to be used, but if it's too narrow I imagine that it would be difficult to use.