Engine wear already happen. No point u use more expensive or better oil. It is waste of money. You will keep burning those 10-15/40 oils no matter how expensive it is. Its either u keep using the current 20/50 and live with it being under powered or go for overhaul. White smoke usually means piston rings are done already.

If no smoke, that means you should have a leak. Engine oil does no dissapear without a trace. Its either burnt up or leaking.