I recently changed out some Delvac diesel oil (either 15W-40 or 10W-40, I forget) for Quaker State's new "Defy" high-zinc oil at a 10W-30 mix.

Yesterday I noticed some light blue smoke out of my left pipe while I was letting it fast idle from cold.

I tried again just now and I still got some smoke, only out of my left pipe. It doesn't start immediately, only after the engine running at 1800 rpm for long enough to get the temp gauge up off the peg. After I knocked it down to regular idle, I was able to get a bit of smoke to appear from revving it but it seemed to go away altogether after a few more minutes. It's never very bad, I only noticed it from the smell at first and then the light haze, and it only seems to be coming from the right bank of cylinders.

I'm not too concerned since this engine doesn't seem to burn oil like it has bad rings or anything (I rarely need to top up more than once every six weeks or so) but I'm wondering where it could be coming from.

Pull the breather on the valve cover and see if there's blow-by, compare with the other side. Probably valve seals, how many miles?

I've only seen valve seal smoke when the engine first starts not after and has run for 1 minute or more. Hi idle has hi vacume I would look at PCV valve then intake gaskets first.

I had a car that started smoking after I changed from Quaker State to pennzoil 20 + years ago I changed back to Quaker State and the smoke stopped I have never used Pennzoil since. From what I understsand now Pennzoil now owns Quaker State or vice versa.

intake manifold gasket issue

Just replace the PCV they only cost $5 unless you can remember the last time you replaced it. If you can't remember spend the $5 and eliminate it as a problem and go from there.