Generally it's caused by the valve stem seals going brittle and cracking (Or just brittle - As what happens to almost all 10 year old rubber bits exposed to hot engine oil!).

Anyhow, the only way that I can see that a material or fluid would fix this is to change the structure of the rubber or somehow clog up the valve guide - someone tell me how this works!?

You can do the seals on the engine, you'll probably drop a valve - yes always move the crank so that the piston you are working on is at the top of it's stroke - we all get carried away when we are working on these things and often forget to turn the crank. Also the most effective way to test the guides is with the head off, no substitute to that one I'm afraid.

Thick oil may well work for a while too - but it is inevitable that this work has to be done really - right? (Do it now and it'll be good for another 10 years - right?)

Burning oil a problem? - as it gets worse you may go through a few spark plugs - if it is really bad. I've driven some really sick engines that would go through a liter of oil in a week (400 miles û thatÆs quite a lot - made a hell of a cloud behind me!), real damage though - not really, the engine runs a little hotter.

Check your plugs - how do they look?