When a bladder-type fuel tank is left empty for a long time, with what should the inside of the tank be coated?

Coating the inside of a bladder-type fuel tank with engine oil is essential for several reasons related to the longevity and integrity of the tank. When a bladder tank is left empty for prolonged periods, the absence of fuel can expose the interior surfaces to air and moisture, increasing the risk of degradation due to oxidation and environmental factors. Engine oil creates a protective barrier that helps to prevent corrosion and other forms of deterioration, maintaining the tank materials' integrity.

Moreover, using engine oil helps in reducing the likelihood of the bladder material shrinking or becoming brittle due to lack of lubrication and exposure to harsher environmental elements. It ensures that when the tank is eventually filled again, the fuel system operates smoothly and efficiently without contamination or degradation of the fuel.

Other options, such as water or fuel, would not provide the necessary protective benefits, and grease may not be suitable for this application as it could trap moisture or not adequately coat the tank's inner surfaces. Thus, using engine oil is the most effective choice for preserving a bladder-type fuel tank when it must be left empty for an extended duration.