I picked up a 120,000 mile 2009 328i over the summer with no service history. The dash said the oil was 35,000 miles old. I immediately changed it to the old style gold bottle Castrol Edge LL01 just to play it safe. The gold bottles 5 quarts go for $50 now. There was sludge around the drain plug. Now flash forward 5000 miles later and the car has not burned a drop! The car runs like new. I want to get the Walmart Castrol Edge black bottle LL01 for $24 this time. I have 3 quarts left over from the gold bottle. The car holds about 7 quarts. Can I mix the gold bottle LL01 with the black bottle LL01 oil? (Also, plan to send off sample to Blackstone this time)