Today, while changing the oil in my 04 S60, I do my normal routine of checking to make sure everything is staying clean and nothing is forming in the oiling system. When I first purchased the vehicle, it had 50K miles on it. The engine wasn't taken well care of, but it was far from what I would call bad. I immediately switched it to M1 0w40 and ran it to the recommend intervals of 7500 miles between changes. However, I switched to GC just to give it a try. I noticed a huge difference in the two oils. M1: -Comes out darker when drained. -Doesn't make the car feel as "held back" -leaves little hard carbon chunks in the oil filter CG: -comes out clear, has a dark red color to it after the whole 7500 mile interval -makes the car feel a little sluggish -does NOT leave any deposits in the oil filter That last line is the one that worries me. Is the M1 cleaning out [censored] from many many miles ago? Or, is it actually turning solid? The pieces are VERY small, about the size of a grain of sand. They are not attached to the walls of the oil filter housing, like I always see on a customers cars that don't get serviced properly. And they wash right out with a squirt of brake clean. Also, there are the same little pieces of hard carbon in the pleats of the oil filter. What is going on here?