The "lubricant compatibility test" that means that any oil can be mixed with any other.
Worthwhile using the actual name of the test
"Standard Test Method for Determination of Homogeneity and Miscibility in Automotive Engine Oils"
What exactly is the raft of testing ?
Visual color determinations and observations are made
on an undiluted test oil specimen, along with six blends of the
same test oil that have been combined with specific reference
oils. The pour point is then determined for the undiluted test oil
specimen and the six blends. The undiluted test oil specimen
and six blends are then allowed to warm to room temperature.
Color determinations and observations are again made on the
undiluted test oil specimen and six blends. The undiluted test
oil specimen and six blends are heated to 232°C, then allowed
to cool to room temperature, and then stored at their pour point
temperatures for 18 to 24 h. The undiluted test oil specimen
and six blends are then allowed to thaw and a series of color
determinations and observations are made as they reach room
temperature. All data are recorded on a report form.
It's not a performance standard, nor does it report that oil A, mixed with oil B exceeds, or even meets, any of the standards that either oil meets for approval or on their own.
The pour point (freezer tipping point) is recorded, and is relevant ONLY for the storage of the oil after it's undergone a heating test to 232C (450F), where it's kept for a day...then thawed out to see...if it's still mixed.
It doesn't qualify that the additives work in conjunction with each other, antagonistically with each other...just that they will stay mixed.
What's the test for ?
It is important that engine oils from different manufacturers be homogeneous and miscible with each other, because operators of automotive engines often do not have prior knowledge of the manufacturer of the oil that is currently used in their application, and engine failure can occur if oils are combined that do not stay homogeneous and function properly
Anyway, next time you are told that oils are compatible, it means that they are miscible...here's a link to the standard.http://www.hdutil.com.br/site/arquivos/0%20forum%20yahoo/D%206922%20%2003%20%20_RDY5MJI_.pdf
THE GF6 Committe made this statement when they referred to ASTM D6922
Homogeneity and Miscibility, ASTM D6922
Shall remain homogeneous and, when mixed with TMC reference
oils, shall remain miscible.
Note, that's 6 reference oils, and not any two off the shelf oils that you may encounter.