1999 Dodge Van Burning Oil

Hello our church has a 1999 dodge maxi wagon 3500 van. This past Sunday we drove to a youth event that was 45 miles away. It burnt 2 quarts of oil on the way there but hardly burnt any on the way back. You can tell it is coming out of the tail pipe because the inside and outside of the tail pipe is charcoal black. A mechanic did a compression test on it and the pistons were at the right level but some how it is burning oil. The mechanic told us he would have to tear the motor down to find out how it is burning oil. We put hyper lube in there to prevent smoking and whatever else it does. Do you have any ideas? We cannot figure it out! It burns oil just not all the time!