ASTM D3339 Test Method for Acid Number of Petroleum Products
ASTM D3339 Standard Test Method for Acid Number of Petroleum Products by Semi-Micro Color Indicator Titration
8. Preparation of Used Oil Samples
8.1 Strict observance of the sampling procedure described in 8.2 is necessary, since the sediment itself is acidic or basic or has adsorbed acidic or basic material from the sample. Failure to obtain a representative sample causes serious errors.

8.2 Heat the sample (Note 7) of used oil to 60 ± 5°C in the original container and agitate until all sediment is homogeneously suspended in the oil (Note 8). If the original container is of opaque material, or if it is more than three fourths full, transfer the entire sample to a clear glass bottle having a capacity at least one third greater than the volume of the sample. Transfer all traces of sediment from the original container to the bottle by violent agitation of portions of the sample in the original container. After complete suspension of all sediment, strain the sample or a convenient aliquot of the sample through a 100-mesh screen for the removal of large contaminating particles (Note 9).

NOTE 7 - When samples are visibly free of sediment, the heating procedure may be omitted.

NOTE 8 - As used oil can change appreciably in storage, samples should be tested as soon as possible after removal from the lubricating or test system and the dates of sampling and testing should be noted.

NOTE 9 - When samples are visibly free of sediment, the straining procedure may be omitted.