Aircraft oil tanks are normally checked at the same time the fuel tanks are filled. There are a few exceptions to this general rule, since some manufacturers recommend that the oil level in certain jet engines be checked within a specified time after engine shutdown. In all cases, the manufacturer's instructions should be followed for the specific aircraft not only for servicing procedures but also for type and grade of oil used.
Aircraft oil tanks should never be filled to capacity or above the labeled full mark on the gauge or dipstick. This is because oil expands when it becomes hot, and at high altitude it bubbles and expands. The extra space in oil tanks allows for expansion and prevents overflowing.
The aircraft's oil requirements should be checked, and no substitutions should be made for the type of oil to be used unless substitute oils have been approved for use. When pouring oil into the tanks, be sure that cleaning rags or pieces of rag or other foreign substances do not get into the tanks. Foreign material in the oil system restricts the flow of oil and can cause engine failure.
Lubricating oil is nonexplosive, very difficult to ignite in bulk, and is not normally capable of spontaneous combustion. However, if oil is ignited, a hotter fire results than that from gasoline. The vapor of the oil, however, is explosive when mixed with air in certain proportions. Vapors of many petroleum products are highly toxic when inhaled or ingested. It is therefore necessary to take all precautions when handling lubricating oil.