14 CFR § 27.1013

It can withstand, without failure, each vibration, inertia, fluid, and structural load expected in operation;

[Reserved]

Where used with a reciprocating engine, it has an expansion space of not less than the greater of 10 percent of the tank capacity or 0.5 gallon, and where used with a turbine engine, it has an expansion space of not less than 10 percent of the tank capacity.

It is impossible to fill the tank expansion space inadvertently with the rotorcraft in the normal ground attitude;

Adequate venting is provided; and

There are means in the filler opening to prevent oil overflow from entering the oil tank compartment.