Do multigrade oils increase valve sticking in piston airplane engines? — General Aviation News

By Ben Visser · December 19, 2022 · 11 Comments

Over the years I’ve found that any change a pilot notes when switching from their usual single grade engine oil to a multigrade oil is usually blamed on the new oil.

This includes — but is not limited to — early cam failures, ruined rings, overheating, and increased rust.

Usually, there may not be a real technical justification to explain the change, but more of a perceived change.

But recently an engine rebuilder called with a question I had not heard before. He asked if multigrade oils would increase the possibility of valve sticking in piston aircraft engines.

My first response was "why would you think that?" Then I started to think through the mechanics of the problem.

Intake valve sticking can usually be related to oil flow down the intake valve stem. The oil flow plus the elevated intake valve stem temperature can result in coking and the formation of hard carbonaceous deposits on the stem and guide area. This, in turn, can lead to valve sticking.

So, the thinking is, the more flow down the stem the more possibility for intake valve sticking.

And one of the big features of multigrade oils is they leak better than single grade oils.

This is where the story gets muddy because of the improved flow characteristics of multigrade oils. The oil ring scrappers tend to work more efficiently at scrapping the oil film off the cylinder walls. This usually results in a 30% to 50% reduction in oil consumption past the rings when a multigrade oil is used.

However, the improved oil flow characteristics of multigrade oil will usually increase the oil consumed through leaks. The oil passing through the intake guides acts like a leak, so more oil consumption.

This is especially true for Lycoming engines with the valves below the rocker shafts, which means more oil in the guide area.

The effect of flow characteristics can cloud the oil consumption debate, with many pilots reporting increased oil consumption when switching to a multigrade, while others report a decrease or no change. Even a report of no change in oil consumption can mean that the decrease in consumption past the rings offsets the increase past the guides.

The increased flow past the guides is compounded by the increased amount of the viscosity improver (VI) additive used in multigrade oils. These complex molecules can be broken down under high temperatures and form deposits.

Before you run out and switch to a single grade oil, you need to know this is not a major problem for everyone. It usually only affects engines running at high temperatures.

Thousands and thousands of aircraft have been run on multigrade oils with no problems.

However, this is just one of the reasons I usually recommend multigrade oils in the winter months and then switching back to single grade oils during the hot summer months.

I wonder if, in the future, all of the problems with aircraft piston engines will be blamed on having to switch to unleaded fuels?