When selecting a lubricant for your aircraft, it is absolutely essential to select the correct one - if the wrong aviation lube is chosen, the components of your engine will likely be subject to increased wear and internal corrosion.
Aviation Oil Weights, or viscosities, come in two distinct types: Single-Grade and Multi-Grade.
Single-Grade Oils may be straight mineral oil (generally used for break-in of aviation engines), or petroleum-based oils with unique additive packages for wear protection and extended use. Single-grade, straight mineral oil is recommended almost exclusively for engine break-in, where controlled levels of friction is actually desirable, but only for a short time (usually from 10-24 hours). After this point, these oils should not be used for flight. Single-Grade lubricants with additive packages are commonly used for in-flight lubrication due to their enhanced wear protection - these oils generally run thick and have excellent cleansing and neutralizing properties. However, these oils are not recommended for use in cold weather, where they may extend start-up times and may be slow to disperse throughout engine components for the first few minutes of operation, leading to increased wear. If heated hangars are an option, these oils are likely a good investment.
If operating an aircraft in cold weather, Multi-Grade Aviation Oil is usually the standard. These oils travel through engine components quickly in cold weather, preventing wear and tear on your engine in the earliest moments of start-up. These oils achieve their increased performance in cold weather by way of Viscosity Index improvers (VI improvers).
Choosing between the two types of lubricants should be based on your flying habits. First, where are you flying? Are you primarily flying your aircraft in freezing temperatures, without a heated hangar? Opt for a multi-weight oil. If it warms up for a few good months, you may want to switch to a single-grade.
If you are flying in moderate climates year-round, single-grade aviation oil will likely provide excellent lubrication all year. If you fly in the less-corrosive environments of the mountains or the desert, you may likely benefit from the use of a multi-weight oil.
As always, change your oil regularly (usually once every few months for normal operation oils) for best results.