The fact that it's missed the services hasn't helped the situation, but I'm thinking of a combination of factors. Quality of oil has an important part, as well as the viscosity. I hope that ultratune hasn't been putting 20w50 in it or something like that. Just a wild guess, that poor quality oil has passed its used-by date and sludged up, not just around the oil filler cap (more to see if you ever remove the rocker covers) but the sludge has likely gotten into the bores and fouled up the piston rings, causing blue smoke and oil consumption.