Oil temperature vs. oil viscosity

Reader David Bennett recently wrote in, asking about oil temperature vs. oil grade.

“When oil is rated for a certain temperature range, is that start-up temperature range or the range for most of the intended trip? My Super Cub handbook states SAE 40 is good from 30°F to 90°F, but on the Aeroshell site, it says 0°F to 70°F. Which is right?”

Temperature vs. oil viscosity is important at both start-up and cruise. If the oil is too thick at start-up, it will increase the time from when the engine first turns to when oil reaches critical wear surfaces. This can increase the wear rate and reduce engine life.

The temperature to be concerned with is the ambient temperature of the engine at start-up. If your plane is sitting outside, that is the temperature of your engine. If your plane is in a heated hangar, that is the ambient temperature.

To determine which oil to use, consult your handbook or engine manufacturer. Most engines recommend using a SAE 50 or grade 100 oil for most warm weather conditions. However, on smaller Teledyne Continental Motors engines and some others, a grade 80 or SAE 40 oil is recommended for most warm weather conditions.

The rule of thumb that I would recommend is to use the recommended SAE grade single grade oil for the warm summer months. When most of your start-ups are at or near freezing (32°F), switch to a lower grade or multi-grade oil.

The other concern is temperature at cruise conditions. If your oil is too thick under severe cold, you can experience oil cooler plugging.

However, under warm conditions, you want a thicker oil to ensure that you have adequate oil film thickness in critical bearings and wear surfaces. You do not want to be flying in hot temperatures with too thin of an oil.

This goes back to my recommendation to use the grade 100 oil, or 80 if recommended, for most of your summer flying and then switch to a multi-grade during the winter or colder months. This seems to give the best combination of good starting and best high temperature protection.

PRESERVATIVE OILS

I also have received several questions concerning the use of preservative oils. I recommend the use of one quart of preservative oil for pilots who will be using their planes infrequently, like during the winter months. This is not recommended for long term storage where straight preservative oil is recommended.

While I cannot give exact numbers for storage time on each product, I recommend that, under most conditions, one quart of preservative oil will be good for up to five to six months. If you are going to store your engine longer, use 100% preservative oil.

However, this depends on your weather and humidity conditions. For example, if you live in Arizona where it is very dry, you can extend this, but if you live in humid Florida, six months can be too long.

And remember: These recommendations are guidelines, not absolute limits, so use some common sense.

Ben Visser is an aviation fuels and lubricants expert who spent 33 years with Shell Oil. He has been a private pilot since 1985. You can contact him at Visser@GeneralAviationNews.com.