Why does my car start differently after changing oils?

I had mobil 1 extended performance and it was ok but after 7000 miles my car would stall a little rarely on cold starts. Switched to castrol magnatec and now it really has a boost on a cold start. When its really hot lile 90 degrees, it makes a rattle for a second though but then doesn't do it again for... show more I had mobil 1 extended performance and it was ok but after 7000 miles my car would stall a little rarely on cold starts.

Switched to castrol magnatec and now it really has a boost on a cold start.

When its really hot lile 90 degrees, it makes a rattle for a second though but then doesn't do it again for weeks.

Why?

I decided to stick with 5000 mile oil changes, fyi...but which oil is better?
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