I would take a look at the fuel injectors and maybe some injector cleaner through it. It's my guess that one or two of your injectors are leaking fuel into and washing the cylinder walls down.

Mine would occasionally make a plume of smoke on startup after just driving it - and had an occasional rough idle. I figured out which injector to change on mine when it did the same thing by looking at the plugs.

Although - 1 quart of oil every 800 miles sounds like it could have another issue as well.