During service, neat cutting oils suffer negligible deterioration of quality; their service life is almost indefinitely long, provided they are kept clean and free from contamination. They suffer very little depletion of their additives through cutting action, and quality is adequately maintained by additions of new oil to make up for ‘drag-out’ losses (those caused by oil adhering to both the swarf and the work). Serious leakage of hydraulic fluid or lubricating oil into the coolant system can have a diluting effect, which reduces additive concentration and hence the performance of the cutting oil.