Misfire untraceable!?

I own a '97 Chevy K1500, 5.7L vortec, auto. About 2 years ago I ran out of gas and had to run on 5 gallons of 50:1 mix 2-stroke.  Shortly after I replaced all of the plugs.  A year later cylinders 4 & 6 fouled out with what looks like bird stuff built up.  This time I did a complete tune-up using Delco Platinum plugs (oe), cap, rotor, & wires.  I ran a bottle of fuel system cleaner through and everything was good. Now, my truck started missing at idle just yesterday.  I got a check engine light with a code of "Cylinder misfire" I pulled all of my plugs and checked the cap & rotor.  Plugs 4 & 6 were fouled up again.  I replaced only those 2 plugs and waited until this morning.  Same thing, only this time, I'm getting a "Cylinder 6 misfire" code.  When pulled the plugs appear to have oil on them.  I did a compression check and all cylinders are pushing 180psi, without leaking down... I've been told a few things, like valve seals, clogged injector, but am really on a budget so any advice as to where to go from here would be appreciated.  

Defintiely sounds like the valve stem seals on those cylinders are shot. You're getting too much oil running down the guides and into the cylinders.