10W40 too light?

New engine probably wants a thinner oil unless it was specifically built with higher clearances.
Pressure is measured at the beginning of the oil system.*
Try this for why thicker can leave some of the system without sufficient oil.
Picture trying to push molasses through to the top end.
Even warmed up, not too much is going to get there, even though there's lots of pressure at the beginning.
Now picture doing the same with vegtable juice.
A lot more will flow through.

I know, kindof silly using food products for an analogy, but its lunch time.

*for a big block it can be in the middle when gage is connected at the cross passages.