Improper engine break in.

QuickDart360 said

What's a good weight to run a stock clearance 360 hot smallblock? I've been using VR-1 30 straight weight with good results. However, 30 straight weight is getting harder to come by. I live down south in San Antonio, Texas which summers can be brutal and cold mild to cold but is short lived. 10/40 or say 10/30? 10/30 is thinner than 30 straight I would think?

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Any multi grade oil always starts out at the winter grade. So a 10w30 starts out as a 10 and uses viscosity index improvers so when it gets to 212 degrees F it acts like a 30.

There is never a reason to use a single grade oil. After WWII the VI improvers were available to everyone, making single grade oils obsolete.

The greater the spread between the winter grade and the operating temp grade means it uses more VI improvers, and gets further away from being a Newtonian fluid.