Was looking on eBay and noticed that 2 quarts of 10W-30 TGMO was going for $7 shipped, while 2 quarts of the 5W-30 TGMO was going for $10 shipped. Is this because of lack of demand for the 10W-30 or because the 5W-30 has better base stck / add pack, and just more expensive to produce?