Intake valve stems rarely, if ever, cause a problem. They run far far cooler and there is vacuum on the intake port side of the guide always trying to pull some oil down the guide. Even a "dry" valve stem seal will allow a lot more oil down the intake as compared to the exhaust. There is no steady vacuum in the exhaust port, rather an overlying higher pressure from backpressure in the system so oil does not naturely tend to flow down the exhaust valve guide as with the intake. Plus, the exhaust valves run far far hotter as you would expect.
It is very common for many engines to run distinctly different valve stem seals on the intake and exhaust as well as distinctly different valve guides and valve stem surface treatment. Most all engines have chrome plated exhaust valve stems for scuffing resistence while intakes will be plain or just chrome flashed. I a lot of cases you can run exhaust valve guides with little or no seals due to the lack of vacuum in the port constantly sucking oil down the guide. An intake guide with no seal would be constant blue smoke.
The heat causes several problems. First, it tends to vaporize any oil that is there reducing what little lubricating properties it has. Secondly, it caused the exhaust valve stem to expand more, reducing clearance in the guide. This alone can lead to scuffing if the cold build clearances are not correct. Third, the exhaust valve guide itself runs hotter as well as the parent material it is pressed into. This may be the most important thing, here. The Exhaust valve guide projects slightly into the exhaust port so one end of the guide is running a lot hotter than the other. This cause uneven expansion of the guide and the parent metal surrounding the guide. Uneven expansion means that the valve guide, which is ideally a perfect column, becomes somewhat "banana shaped" to exagerate. The banana shape of the guide when hot ditates the clearance requirements so that the guide will never bind on the stem even when it is hot.
Possibly the left side guides run a little cooler than the right side due to coolant flow patterns thru the head??? Maybe there is some influence of the oil flow inside the head causing a slight temperature gradient across the head as measured at the valve guides. There is always the possibility that the left cylinders run a little leaner than the right cylinders. The O2 sensor measures the sum output of all the cylinders. Some individual cylinders may be a little leaner than others. The O2 and closed loop software just make sure that the OVERALL output of the engine is 14.7:1. Not that any cylinder is likely to be way lean at all but that the left ones might be a few tenths of an air fuel ratio leaner than the right. This would cause a few hundred degrees difference in EGT thus changing the relative distortion of the exhaust guides across the engine....thus causing more wear where the distortion is worst....as you go across the engine to the right side.
Hard to do more than toss out ideas here without a dyno and some measurements and data. A thermocouple in each cylinder would provide a clue if someone wanted to drill their pipes and stick a 4 cylinder EGT system on and do some riding. You would be looking for consistent EGR differences...not a dramatic high or low (lean or rich) pattern.
I have seen situations like that with port fuel systems. Depending on the airbox, the left cylinders may flow more air than the right possibly causing them to be a little leaner for any given pulse width. The location of the fuel pressure regulator on the fuel rail can impact actual fuel delivery thru each injector due to the pressure hammer effect (water hammer) when each injector closes. The pressure pulse can travel to other injectors affecting the amount of fuel they deliver. This can cause distinct rich/lean patterns across a fuel rail. One assumes the engine designer covered this during development.
Injectors are randomly assumed to be plus/minus 3 % for fuel flow rate....so unless the injectors are flowed and matched you can expect a 6 percent range in fuel delivery across the injectors. This would be really rare as generally the injectors are much closer than this and are either all rich or lean....and then the closed loop system corrects for the engine as a whole and there is very little difference between cylinders. The injector effect doesn't explain the left to right issue as the injector issue would be more random.
Just some ideas and things that I have observed. The whole situation with the FJR is not too hard to explain, actually, from an engine development standpoint. They may run a little low on clearance in the exhaust guides (production tolerances) and maybe someone forgot to adequately prelube the valve stems at assembly starting the snowball. Then one side of the head distorts a little more than the other due to some of the reasons mentioned..... Then the valve stem seal supplier retools the seal a little "dryer" by accident thus limiting the amount of oil down the guide that would tend to cover up some of the other issues.
Once again, pure speculation, but it fits my preconceived notions of what could cause the results observed. It is probably not just one thing but an unfortunate combination of circumstances.
