Hi Guy's, Just after a little advice and thought on a few engine problems that may be oil related or perhaps coincidence only. I'll list the problems below, but what got me directly wondering is, that I have been using lets say brand X lately and different products brand X makes in different engines and have had a few problems happen. 1: A 80's model, recently rebuilt Ford Cleveland V8 engine started leaking from the rear main seal when I used Band X's High Mileage oil in it. Could this just be a coincidence? 2: A 86 1.5L Toyota engine, that has done a lot of Mileage. I have noticed that when I used Brand X's thicker oil (was using same weight in other brands before) also suited to racing, found that the Timing Chain become noisier, or even more rattly on de acceleration that before. 3: Now this may just be me imaging, but on the same engine, prior to using there High Mileage product the top part of the engine viewed through the Oil filler cap hole, mostly the Rockers and springs and so on, were just the golden brown oil colour, but now went a dark more dirty looking colour. So could these problems just be shear coincidence? The brand has been around a very long time and I have not heard of anyone else with problems like this.