Synthetics are not a bad idea or a good idea. My understanding is they have better cold flow characteristics and better heat stability but do you need both of those features for the price. I use synthetics in two of my vehicles and dino oil in the other. To offset the price of the synthetics I use longer drains but have relied on oil analysis to make sure the oils are holding up between changes. Synthetics have better cleaning action and will not affect seals unless I guess if dirt is helping to seal any leaks. The 0w-40 synthetic I use in my Montero is not available in a dino oil and I am extremely happy with the way it performs so far. The key to any oil and engine life is keeping it clean. If you have always kept the oil clean then a flush is not necessary. It will also take a couple of oil changes to flush out the dino with the synthetic to see the full affect of the synthetic. You may have to experiment a bit to get an oil you are happy with. I am still analysing the two different brands of synthetics I use and am not committed to either yet. Life is a journey. <img src="/forums/images/graemlins/confused.gif" alt="" />