I bought a 15 RGS with 700 miles on it. The dealer said they already did the 1K service with standard HD oil. So i put about 2k miles on it the last 2 months. The weather here in Tampa is getting hot so i decided to do an oil change with Mobile1 Vtwin 20/50. This is my first Touring bike with an oil cooler . I drained the oil and removed the oil filter. When I measured the amount of oil that drained out, it barely measured 3 Qts. Could there possibly be a Quart still in the oil cooler? I am not sure how much of the new synthetic I should add now? I am also wondering if there would be any issue with the synthetic oil mixing with the standard HD oil. Is there any different i need to do when changing oil on a bike that has an oil cooler to ensure all of the oil drains completely?