synthetic motor oil just happens to have a higher burning(combustion) temperature than that of ordinary dino oil, thus the possibility of lower/less smoke-like exhaust coming out.
The logic goes like this:
(if your engine has oil burning issues)
if previously running dino oil and it smokes-->switching to full synthetic oil
observable outcome: less "visible" smoking due to higher burning tempurature with synthetics.
If prevously running synthetic oil and no "visible" smokes---> switching to dino oil it "smokes" more
this is due to dino oil's lower combustion temperature, while all other things being equal.
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This does not necessarily mean that your valve guides are in good shape. (seals fine means nothing in this case). Excessive clearence on valve guide bore will cause excessive oil leaking past the internal bore, regardless of how "good" your valve guide seals are.
Granted that you do not have any excessive crankcase pressure (meaning that your PCV part is functional and no excessive blowby causing excessive crankcase pressure), your last likely cause should be worn valve guide bores. To properly service them is to send the head to a shop and press in some new ones.
Oh and BTW: old tricks die hard: don't consider "reaming" the internal bore with screw-like ridges just to shrink the internal valve guide bore a bit. This is an old hat's trick that will only lasts approx.3000miles before your engine starting to smoke again.
p.s. plugs may not show any significant signs of oil burning due to extreme combustion temperature (unless your motor oil is of high-ash variety or your engine is burning 1qt every 300 miles).