I have decided to do my oil changes at 4000 miles. I had 2800 miles on my oil, leaving me 1200 miles to go before my next oil change. Coincidentally, I left on a trip for vacation (and am there now). When I get back home, my oil will be at 4000 miles. However, just prior to leaving on my trip, I was 1 quart low and added a fresh quart (Havoline 5W/20). 1 quart is 20% of my Dodge Grand Caravan's oil capacity. By adding a "fresh" quart, by how much am I extending the overall life of my oil? I mostly drive short distances (2-3 miles in cold South Dakota weather). If adding a fresh quart between changes becomes routine, am I safe pushing my OCI to 5000 miles?