14 CFR § 25.1013

Installation. Each oil tank installation must meet the requirements of § 25.967.

Expansion space. Oil tank expansion space must be provided as follows:

Each oil tank used with a reciprocating engine must have an expansion space of not less than the greater of 10 percent of the tank capacity or 0.5 gallon, and each oil tank used with a turbine engine must have an expansion space of not less than 10 percent of the tank capacity.

Each reserve oil tank not directly connected to any engine may have an expansion space of not less than two percent of the tank capacity.

It must be impossible to fill the expansion space inadvertently with the airplane in the normal ground attitude.

Filler connection. Each recessed oil tank filler connection that can retain any appreciable quantity of oil must have a drain that discharges clear of each part of the airplane. In addition, each oil tank filler cap must provide an oil-tight seal.

Vent. Oil tanks must be vented as follows:

Each oil tank must be vented from the top part of the expansion space so that venting is effective under any normal flight condition.

Oil tank vents must be arranged so that condensed water vapor that might freeze and obstruct the line cannot accumulate at any point.

Outlet. There must be means to prevent entrance into the tank itself, or into the tank outlet, of any object that might obstruct the flow of oil through the system. No oil tank outlet may be enclosed by any screen or guard that would reduce the flow of oil below a safe value at any operating temperature. There must be a shutoff valve at the outlet of each oil tank used with a turbine engine, unless the external portion of the oil system (including the oil tank supports) is fireproof.

Flexible oil tank liners. Each flexible oil tank liner must be approved or must be shown to be suitable for the particular application.