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Yep, slight angle - but shouldn't matter if the "IN" is pushing oil to the
"OUT" then the air would move right along with it and out of the cooler.

Joe Hull
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Joe,
it can matter if the "OUT" is pointing down.

Depending on the cooler design you might have a compressed air cusion on top
of the oil crossflow.
BMW had a similar problem with oil-coolers on their Boxer motorcycles in the
early 80's.
They used a special procedure to fill/mount them.
Most owners who where savy enough, turned the cooler upside down, which
entailed 2 180º fittings and less elegant (optically) solution, but it was
self-purging.....

TJ

PS: If your "OUT" is on top, the problem lays somewhere else....