Possibly people who told you that the problem of white smoke comes from piston rings is right.
I explain myself, when an engine already has a certain life, the piston segments or rings are wearing the continuous
rubb It was new, these vapors are reabsorved by the engine and burned causing the white smoke burned oil.
2º That the engine oil that is intended to grease the cylinders and that is deposited as the piston rises, is not collected as the piston low, burning when the combustion of gasoline occurs and causing the aforementioned smoke.
You have not completed how many kilometers the minivan has, but it may be time to repair the engine.
Another point to consider are valve seals, which as they wear out are letting over excess oil towards the valve offspring and it ends up burning as in the case of segments.
All the best.