Now at 12500 miles I have run Red Line 10W30 since 1000 miles in an '03 Corolla 1.8L 5 speed manual. Two changes during the interval, one @ 6K the next @ 12K. Recently I changed to 10W30 Mobil 1 due to price and availability. Mind you, I thought nothing of it when changing from one synthetic to the other and had no presumed expectations other than saving a few $$. What I noticed immediately after changing to M1 and starting the car was how much quieter the engine was. This "hushed noise" was also noticeable on those days when you have got to get with it to merge even though the engine is still cold. With the RL the engine protested with a rattle (valve train, pistons?) at about 3000 RPM. Now there is no noise at all. This is not my common practice to push a cold engine but we all occassionally have to stand on it to blend into traffic, cold or not. Could this silence of the limbs be traced to the RL being on the Thicker side of 30 weight and M1 on the thinner side? The change is definite but I'm curious as to why.