Hi Mark S.,

Sorry to hear of the high oil consumption in your engine.

 >> Now from what I understand, oil doesn't leak past the valve stems,
 >> because they're horizontal.

Well... actually, in real life... worn intake valve guides and/or
hardened valve guide seals, can certainly provide the clearances to
cause oil consumption.

You will notice bluish smoke from your exhaust upon coast conditions
(i.e. high vacuum), which is a reliable indicator of this normal pattern
of engine wear.

Sounds like it could be time for a valve job, if some other cause for
the oil consumption doesn't present itself.

Later
Derek -