Have a mechanic do the leak test, check all your pistons are at right compression. Do you run ECU or AFC? Also your spark plugs are alright. Another is there is a possibility that the valve stain seal could be damaged. It's the seal that blocks out the air/oil when the pistons are working. You could alternatively try using thicker based engine oil, and see if the white smoke still comes. Most likely, is the pistons are leaking oil. check the compression.