I have an '04 4Runner V8, do mild towing, mild off roading, and sometimes drive in extreme temperatures (100 degrees in summer, sometimes minus 20 in the winter). For the first 44,000 miles I used the recommended 5W30 with no problems whatsoever. I just bought some new tires and was offered a free oil change with Kendal oil through Tiresplus (an excellent tire store by the way). I was going to go with synthetic but they only had 10W40 available in synthetic. I chose to go with Kendal synthetic blend in 5W30 in a 50/50 blend because Toyota recommends 5W30 for this vehicle. As it is explained to me a synthetic blend offers the benefits of synthetic at a reduced cost. Here are some questions I cannot get a straight answer on: 1) In the past it was said that once you go to synthetic you cannot go back to dino. I have read that that is not accurate. Can anyone confirm or deny this fact? 2) Is a blend as simple as a portion of synthetic and dino? If so, can you top off a dino with synthetic or top off a synthetic with a dino? It would seem to me that an engine doesn't care what brand or whether it is synthetic or natural as long as it retains is lubrication and heat absorbing properties efficiently and is clean. Am I incorrect in thinking that you can use synthetic and natural oils interchangably?