I just did my first oil change is this car I got second hand. the manual says it can use 15W 40 or 50. i was able to find 15W 40 (shell helix fron true value). It says it's designed for older cars.

I noticed lang that I can here the tappets more with this thinner oil. Is this ok or a sign that I'm using too thin an oil?

BTW, the car is 7 years old and has 70K+ kilometers on it.