"Diesel fuel maintenance and regular testing should begin the day of installation and first fill to establish a benchmark guideline for future comparison.  Diesel fuel should be monitored for water on the bottom and tested for degradation and contamination."  It is recommended that the fuel should be tested at least annually using applicable ASTM standards OR the manufacturers recommendations.  If a fuel is found to be out of spec then it should be tested every 90 days until the analysis is deemed acceptable.

http://machinerylubrication.com/Read/23920/oil-sampling-lists

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Thankfully, in the realm of oil analysis and machinery lubrication, few do’s and don’ts can be considered subjective. In this case, we’re talking about what to do and not do related to oil sampling for analysis. These simple rules will make or break the integrity of your sample, which is meant to drive your maintenance and reliability decisions.

Machinery Lubrication Magazine

http://www.machinerylubrication.com/Read/30613/eng...

"Where and How Often to Perform Oil Analysis

With most oil analysis of industrial plant equipment, a sample is collected every “X” number of weeks or months and evaluated using a predetermined set of routine tests. This is practical when the cost of oil extraction and replenishment is reason enough to extend the drain interval until it has reached its condemning limit and when pulling a sample during operation is effortless because of an installed sample port. Even if it isn’t costly to replenish the oil, it may be important to monitor machine conditions through oil analysis simply because the machine is critical."

The test that Lab One Ins suggests to their clients are: