Recently changed to synthetic oil, Syntec 5W50 from Castrol 20W50. 77 280z has 37k original miles. Engine has good compression and works well. But, has started to show a little oil smoke when rpm > 4500, none below. Oil consumption is 1 liter (qt) /2500 miles.

Switch to synthetic probably losened some dirt/varnish that has caused the oil rings to stick allowing some high rpm blowby. Pcv valve is good.

Considering an engine flush to free up/clean oil rings.

Any experience or recommendations re my situation/strategy??

Thanks!

Here's my guess... Since it is low miles, but has age against it I would guess you are sucking oil past the valve guide seals since synthetic is a lot slipperier and only 5 weight. That and the fact that it only happens above 4.5K, there's a lot more oil on top then than at lower r's. I'd install new valve guide seals and try again, cheap fix and it wouldn't hurt.

Lance,

Your diagnostics are good.

I forgot to mention that the head was professionally rebuilt last year with new valve guides and seals when the old head gasket failed and needed to be replaced.

Don

Close but no cigar!!

Well Don, you're making it tougher Since the head is rebuilt that can pretty much be eliminated. Seems strange that it's only at hi r's, if it was rings I would expect it all of the time. You can try an engine wash, I have no experiance with them so don't know if they really work. If you try it and it doesn't help I would probably switch back to regular oil. Eventhough you are not burning that much I wouldn't want to be adding that carbon to the combustion chamber along with the added plug fouling.