For the oil in a pressurized lubrication system, oil samples for analysis and trending should...

The center of flowing oil line upstream of the oil filter, to assure that the sample is representative of the oil leaving the lubricated components.

The center of a flowing oil line downstream of the oil filter, to assure that the sample is representative of the oil being supplied to the lubricated components.

A low point in the oil sump, to assure that the sample includes wear particles, water, and other contaminants that may have settled out of the oil.

The oil sump at a point above the bottom, to assure that the sample does not include wear particles, water, and other contaminants that may have settled out of the oil.