Oil + additive viscosity

Thanks! very interesting.
Sorry I missed that you posted last night.

While it wouldn't hurt opening up any of the smaller holes if there's enough meat, or chamfering the sharp edges, I don't think that's the real issue.
The oil pump exit passage and the right angle adapter passage were less than 1/2" originally. Here's Clair's webpage where he posted dimensions on a variety of factory adapters and bolts. So this why I don't think the I.D. is the cause.
340 Right Angle Oil Filter Adapter

I chamfered the inside of the both the casting and long bolt on this one.