620Hp small block

MACKIN said

If you run synthetic IMO you'll be fine. The lower number is for cold starts, higher for better protection (viscosity) under load hot. Best to really ask a pro builder.

What are you worried about fuel economy? LOL

Newer vehicles have gone to lower viscosity for fuel economy and tighter tolerances is the way I understand it. Foe example most cars today call for 10w (or less) 30. IMO it's mostly for fuel economy and they can run lighter oil for tighter (longevity) tolerances in the engines.

But What Do I Know!

Click to expand...

Lighter weight oil flows better when cold. Cold start ups are the hardest on an engine because there is no oil flowing to critical areas. The most important difference between synthetic and mineral oil to me is how they get their viscosity. A 10w30 mineral oil starts out as 10 weight and the additive package modifies it too 30 weight when hot. So when the additive pkg wears out it stays at 10 weight when hot. Synthetic 10w30 starts out as 30 weight and additives decrease it to 10 weight when it's cold. So even if the additive package wears out it's never too thin.

But even at 10 weight it's still too thick to properly flow through an engine at even 75 degrees. I'm using 5w20 synthetic in my race engine because I get it for free but I'd rather use a 0w.