Stainless, Bushed PRW Rocker Arms

I don't think that solid or hydraulic dictate whether grooves are needed. The rocker is always riding on a film of oil, otherwise the system fails. I don't see how either one can be substantially better at getting oil to the interface, especially when you consider the time available to move the oil. The truth of the matter is that the groove distributes oil more evenly across the rocker so that the oil film has a better chance of standing up to the load. Just about every plain bearing application I've ever run across in industry ( punch presses, lathe and mill spindles, linear slides, etc ) all have grooves machined across the inner face of the bearing to distribute the oil. It may even be more critical in a system with reciprocal motion like the valve train, than a rotary one. I am not debating whether a plain bearing without grooves can work in a given application, but I will guarantee you that the one that has proper distribution grooves in the bearing will outlast the other one ten out of ten times.
I've got a set of PRW 1.6 rockers that will be going in this summer (hopefully). I've cleaned a ton of crap out of the passages, chased all the threads, and will be grinding grooves into the shafts before they are installed. I can't remember if mine are one or two piece bushings, but in my mind the two piece is a better design because it is has a larger available volume of oil available local to the wear interface. May not make a significant difference but it can't be a bad thing.