Take it back over to them and have them check it out. there aren't many ways for oil to get in there: blow by from the valve cover to the intake, vlave guides, valve seals, a crack to an oil passage. The first would not likely give you a startup smoke and the last is way remote. If its guide / seals, put the top down do 70 on the freeway and coast down to maybe 50 on an offramp. That sucks oil down the guides into the valves. Then accelerate quickly. You would see a puff of blue smoke from the oil in the intake. You might have some one following to see it.
