No easy way to check that out or fix it is there?
Haven't done it on these engines but on others I've worked on, the head has to be removed and the guide has to be replaced. If the guide is not a replaceable part, then the guide can be reamed out & a bronze "sleeve" is pressed in. Even with new guides, the operating clearances have to be verified between each valve & its respective guide. Too much clearance & the valve will not run smoothly in the bore, creating more accelerated wear & also more clearance causes higher temperatures on the valve, due to poor heat transfer to the head.
Another option & I don't know if they're available or if another (non-Onan) part # would work, but you can also do a "band-aid" by installing valve stem umbrellas that slip over the valve stem on top of the existing guide. IIRC, they used to be made from Teflon. This can be done in place by applying compressed air to the cylinder so the rocker arm & valve spring can be removed while the head remains on the engine. The down side of this is that you're not restoring the proper operating clearance between the guide & the stem but instead "re-routing" the oil around the guide area. Also, with this repair, the oil leakage/burning problem is fixed but the valve is allowed to "wobble" in the bore of the guide, which sooner or later will require attention, depending on the wear amount on the guide.
Some guys used to buy valves with over-sized stems (~0.001-.003") & just install them in the existing guides (fitting still required), but obviously the head requires removal to do this--whether you change guides or valves....