My wife's Toyota Rav4 has always had a slight timing chain rattle on start-up. It's not a real chain clatter but just a noticeable chain noise. It has always been serviced with dino 10w30, most of the time genuine Toyota 10w30 from the dealer. I decided to change the oil myself this time and used Mobil Super 5w30 semi synthetic and to my amasement it has totally cured the start-up noise. This is the only time I've noticed an oil having any noticeable effect on an engine at all.