What is the science behind changing oil by calendar vs Hobbs?

[Bill Boyd]

Worth the watch, IMO, Gasman. Thanks for posting that.

My Stihl stuff is still getting the 30 year old Pennzoil 2-stroke Air-Cooled 50:1 stock until it's used up. Hopefully they won't have hard feelings about being treated as second-best, but 2-strokes will always be the red-headed stepchild around here...

Clearly the home testing he does here isn't anything close to quantitative science. The line between okay and not okay to use in an engine is rather subjective even if we did have a way to quantify it. Acceptable wear rates, risk tolerance, and economics of maintenance versus overhaul would all (for me) factor into deciding how good an oil has to look to be serviceable. At the end of the day, cutting an oil can open and hitting it with a flashlight beam and a dairy wisk will require a very subjective guess as to what results are acceptable, so I'm not sure the video advances our best practices in the field even if it helps clarify the reasoning behind safe storage interval recommendations.

Storage in a sealed bottle and in an operating crank case are also in no way comparable.

My gut, for which I have no credible defense other than personal opinion and preference, is that my personal minimums for calendar oil change interval is closer to 12 months than 4, if the Hobbs doesn't hit 50 hours first. That's assuming I don't start with oil that's been years on the shelf before it's used.

 

[Freemasm]

........My Stihl stuff is still getting the 30 year old Pennzoil 2-stroke Air-Cooled 50:1 stock until it's used up. Hopefully they won't have hard feelings about being treated as second-best, but 2-strokes will always be the red-headed stepchild around here...

Sorry to step in it, Sir. This is your thread.

Do you think the reference to 2-stroke oils has a basis here? Once through and consumed is not nearly the same application/conditions/duty cycle here. That and the relative costs of consequences between your lawn equipment and aircraft PP?

Have been watching this hoping one of the ChemEs from the industry would chime in.

Side note drift, sorry. Stihl makes some quality as you know. The cheap, home-store stuff has an 8 hour design life (~one season for most home owners) per one of their producers. A 2-cycle carb replacement is 80-90% of a whole new item. No need for anyone to reply here with their exceptional experiences.

 

[RV6_flyer]

Another video but this one does have lab test results of old and new oil.

 

[Bill Boyd]

Sorry to step in it, Sir. This is your thread.

Do you think the reference to 2-stroke oils has a basis here? Once through and consumed is not nearly the same application/conditions/duty cycle here. That and the relative costs of consequences between your lawn equipment and aircraft PP?

Have been watching this hoping one of the ChemEs from the industry would chime in.

Side note drift, sorry. Stihl makes some quality as you know. The cheap, home-store stuff has an 8 hour design life (~one season for most home owners) per one of their producers. A 2-cycle carb replacement is 80-90% of a whole new item. No need for anyone to reply here with their exceptional experiences.

No problem, my good fellow. I enjoy hearing everyone's thoughts, even if there's a paucity of hard data - we're all in that boat together. 2-stroke is indeed a different animal - as you pointed out the oil is burned in use and the crank case is always empty. I hope Stihl is resilient enough and 30+ year old oil is stable enough (and perhaps no significant additives to settle out in storage?) to give me more years of rather heavy farm and home use. Irrelevant to Lycomings and RV's, for sure.

I am loving my RV-10 but not flying it as much as I anticipated - it's a traveling machine and it does that every well. I don't take it up for near as many 0.3 hour cruises around the patch just to scratch the itch as I did when I had the 6A. 40 hours per year is not a lot of air time for a plane that sits literally in the back yard. I probably mow my airstrip twice for every time I fly from it, at least during the grass-growing season. Just did it again today I've been justly accused by the Saucy Wench of mowing grass for the sheer pleasure and relaxation it affords - a chance to get lost in a sea of green grass out to the horizon and the drone of the Kawasaki... I won't deny how nice it can be. Given my flying habits, the calendar versus Hobbs oil change recommendation hits close to home - thus this thread.

Carry on.

 

[Bill Boyd]

Another video but this one does have lab test results of old and new oil.

We need to get these guys to run the same chemical and foaming analysis on new Phillips X/C and AeroShell vs. same with 50 hours run time and 4 months of age versus a year (or more) of calendar time. Wonder what that would cost?

 

[RV7 To Go]

So this is what Aeroshell has to say about oil change frequency.

"OIL CHANGE INTERVAL
Almost all oil change recommendations specify not only an engine hour time limit, but also a calendar time limit. On low usage aircraft the calendar time limit is usually more critical than the engine hour limit. The need for frequent oil changes in aircraft is not caused by the oil wearing out, but rather by the oil becoming contaminated with by-products of combustion, dirt, water (both atmospheric as well as from condensation inside an engine) and unburnt fuel. This contamination can cause corrosion in the oil wetted areas of an engine and thus changing the oil removes these contaminants and helps to minimise corrosion. In order to minimise this corrosion inside low usage engines, calendar time changes are important."

They also recommend lab testing of oil in sealed containers before use if it is 4 years old, assuming proper storage (away from sunlight and heat), and every 2 years after the initial 4 year test from the manufacturing date.

From the ACS site the following regarding the Phillips XC 20-50.

"According to Phillips tech support, there is no set shelf life for this oil as long as it is stored properly. After 5 years, they recommend the oil be retested prior to use."

I fly about 100 hrs/yr. So 2 oil changes per year or about 6 mths per change. My engine is filtered. I also use CamGuard and a desiccant drier. I feel this is adequate engine and oil protection and close enough to the Lycoming interval requirement. I use Phillips and don't have it around long enough to go past the recommended testing date. YMMV

 

[Bill Boyd]

So this is what Aeroshell has to say about oil change frequency.

"OIL CHANGE INTERVAL
Almost all oil change recommendations specify not only an engine hour time limit, but also a calendar time limit. On low usage aircraft the calendar time limit is usually more critical than the engine hour limit. The need for frequent oil changes in aircraft is not caused by the oil wearing out, but rather by the oil becoming contaminated with by-products of combustion, dirt, water (both atmospheric as well as from condensation inside an engine) and unburnt fuel. This contamination can cause corrosion in the oil wetted areas of an engine and thus changing the oil removes these contaminants and helps to minimise corrosion. In order to minimise this corrosion inside low usage engines, calendar time changes are important."

They also recommend lab testing of oil in sealed containers before use if it is 4 years old, assuming proper storage (away from sunlight and heat), and every 2 years after the initial 4 year test from the manufacturing date.

From the ACS site the following regarding the Phillips XC 20-50.

"According to Phillips tech support, there is no set shelf life for this oil as long as it is stored properly. After 5 years, they recommend the oil be retested prior to use."

I fly about 100 hrs/yr. So 2 oil changes per year or about 6 mths per change. My engine is filtered. I also use CamGuard and a desiccant drier. I feel this is adequate engine and oil protection and close enough to the Lycoming interval requirement. I use Phillips and don't have it around long enough to go past the recommended testing date. YMMV

Interesting. I wouldn't think there was enough oxygen exposure in "oil-wetted areas of the engine" to support corrosion, but maybe there's a non-oxidative attack on the metals that we need to prevent.

 

[Snowflake]

Almost all oil change recommendations specify not only an engine hour time limit, but also a calendar time limit. On low usage aircraft the calendar time limit is usually more critical than the engine hour limit. The need for frequent oil changes in aircraft is not caused by the oil wearing out, but rather by the oil becoming contaminated with by-products of combustion, dirt, water (both atmospheric as well as from condensation inside an engine) and unburnt fuel.

I wonder whether they are talking about extended calendar time *in addition to* extended run time. Oil could only become more contaminated with by-products of combusion, etc. through more use... i.e. if more combustion was happening. I suspect most of us aren't trying to run an engine beyond 50 hours (with a filter), but some of us may run less than 50 hours a year. If you fly <20 hours a year, you're introducing the same amount of combustion by-products each time you do fly, but it would take about a 3 year period to get to 50 hours.