Why isn’t there a group III base oil type lubricant for piston powered aviation engines?

[2.7ecoboostFordBronco]

Piston aviation engines by and large still require leaded fuel. This allegedly poses a problem for certain synthetic oils, for instance, Mobil AV-1 which was, as was, as I understand it, a mostly PAO/group iv base oil formulation. Reputedly, the oil could not adequately suspend the lead salts from 100LL avgas (Saavy Maintenance). As a consequence it was pulled from the market ~30 years ago. The Mobil AV-1 story

Piston aircraft engines also are mostly air cooled and operate with cylinder head temps in the 300-420F range, significantly higher than most cars, requiring an “ashless dispersant” (or non metallic?) additive package and limiting detergent utilization. I have been told the lead isn’t a base oil problem, but an additive problem, however the market seems not to have addressed it.

Exxon Elite was reportedly a 25% PAO blend but was also discontinued. As far as I know, Aeroshell W15W-50 is the only synthetic blend on the piston aircraft market and it is reportedly a 50% PAO base oil formulation. Everything else appears to be a mineral oil base, ashless dispersant formulation.

Wouldn’t a group iii base ashless dispersant be able to suspend the lead from the fuel just as well as a group i/ii and potentially better than a iv? Would a group iii have properties that make a higher viscosity index possible than ii? Presently 15W with Aeroshell W15W-50 is the best winter grade commonly available.

Is the market too limited and liability too great a risk for any of this to be considered? Or are there chemistry reasons why it hasn’t, to my knowledge, been marketed?

 

[Nessism1]

Wouldn't changing oil chemistry trigger the need for recertification of the engine in question? If so, that would cost big bucks.

 

[JavierH19]

Wouldn't changing oil chemistry trigger the need for recertification of the engine in question? If so, that would cost big bucks.

If the engine manufacturer changes an oil specification and more importantly maintenance/service guidelines maybe but for the most part I don't think so.

 

[JavierH19]

Locomotive diesel oil and delo adf 600 are similar in that regard. Mostly ashless and have no zinc or phosphorous. It doesn't face the challenges of leaded avgas but are still synthetic blend at most. From what I know it's because synthetic bases are less reactive because they're more linear and uniform or smooth in structure and can't absorb and suspend byproducts while the opposite is true for group 1 and 2. No surprise piston engine oil is still conventional.

 

[johnmyster]

Locomotive diesel oil and delo adf 600 are similar in that regard. Mostly ashless and have no zinc or phosphorous. It doesn't face the challenges of leaded avgas but are still synthetic blend at most. From what I know it's because synthetic bases are less reactive because they're more linear and uniform or smooth in structure and can't absorb and suspend byproducts while the opposite is true for group 1 and 2. No surprise piston engine oil is still conventional.

Speculation around ADF600 is that it's marketed towards fleet use, where soot load ultimately dictates interval and the advantages of a full syn base (high temperature and/or extremely long interval stability) would never be realized. Thus, a blend using grp II is still more than adequate. As such, the product spends it's budget on the novel (proprietary and in-house) additive package and saves on base to keep to a fleet palatable price point.

EDIT - I think you asked two questions:
1) Why not use grp III? I'll let the MO experts answer this, but perhaps aviation oils are using grp iii but have not adopted the full syn marketing terminology. (Sounds like "full syn" has a black eye in aviation space.)
2) Why not a push to "better" base stock oils? Like the ADF600 product in the fleet market, perhaps piston aviation maintenance intervals are sufficient to make re-invention of the wheel unnecessary.

 

[2.7ecoboostFordBronco]

Speculation around ADF600 is that it's marketed towards fleet use, where soot load ultimately dictates interval and the advantages of a full syn base (high temperature and/or extremely long interval stability) would never be realized. Thus, a blend using grp II is still more than adequate. As such, the product spends it's budget on the novel (proprietary and in-house) additive package and saves on base to keep to a fleet palatable price point.

EDIT - I think you asked two questions:
1) Why not use grp III? I'll let the MO experts answer this, but perhaps aviation oils are using grp iii but have not adopted the full syn marketing terminology. (Sounds like "full syn" has a black eye in aviation space.)
2) Why not a push to "better" base stock oils? Like the ADF600 product in the fleet market, perhaps piston aviation maintenance intervals are sufficient to make re-invention of the wheel unnecessary.

I don’t think there is a group iii, it’s really just Phillips and Aeroshell on the market. A Phillips rep stated a few years ago that they would never use a “synthetic”. I don’t think Aeroshell has changed their formulations much since the 90’s, and that was before group iii was very widely used as far as I know? So they mostly use ashless dispersants with a group ii base or a PAO blend.



As far as maintenance intervals, for the owner flown private fleet at least there is no regulation about engine oil changes. I do mine after 25 hours and engine manufacturers recommend that you change the oil every 4 months or 25-50 hours. The lead in Aeroshell W15-W50 gets close to 5000 ppm if I try to go much over 40, and I don’t like that.

 

[RDY4WAR]

A majority of people in aviation think synthetic is bad, often based on a misunderstand issue from a synthetic oil that hasn't been produced in decades and had nothing to do with the base oil type. (was an additive issue) Even if a brand finds better results with a group III base oil (which they would and do), Joe Airdawg McGraw ain't gone buy it. They produce what will sell, not necessarily what works best. Then because the brands keep putting out only conventional, this affirms the myth that conventional is better, and the world keeps getting a little dumber. It's not science, it's marketing and profits.

 

[2.7ecoboostFordBronco]

A majority of people in aviation think synthetic is bad, often based on a misunderstand issue from a synthetic oil that hasn't been produced in decades and had nothing to do with the base oil type. (was an additive issue) Even if a brand finds better results with a group III base oil (which they would and do), Joe Airdawg McGraw ain't gone buy it. They produce what will sell, not necessarily what works best. Then because the brands keep putting out only conventional, this affirms the myth that conventional is better, and the world keeps getting a little dumber. It's not science, it's marketing and profits.

Can you elaborate on what the additive issue was with Mobil AV-1? I might not understand your answer but I’m just curious. It seems a shame that oil for small airplanes evolved this way.

 

[MolaKule]

This topic was discussed and answered previously:

https://bobistheoilguy.com/forums/search/4228688/?q=Mobil+AV-1&t=post&o=relevance

2

Post in thread 'What oil "clings" best to an infrequently run engine?'

Apr 20, 2025

It's possible the reason conventional is still favored in aviation oil is simply due to customer opinion or theory. Companies (especially the majors) produce products that will sell, not necessarily what works best. Even if P66 found much better results with a group III synthetic, the fact that 95% of their customers (in that realm) think synthetic sucks means they're more likely to use a conventional base oil, that they know is inferior, because it's what will sell. Then because the company keeps making only conventional, that further validates the myth that conventional is better...

• 2.7ecoboostFordBronco

7

Post in thread 'Ashless Dispersants in Gas Engines'

Oct 17, 2023

Aeroshell 15W50 is semi-synthetic, as compared to Phillips XC 20w50 which is mineral oil. Mineral oils can hold contaminants in suspension better than synthetics, which is important when running leaded gas in air-cooled engines (maybe also in gas powered air cooled magneto fired lawnmowers). Years ago, Mobil had a synthetic aviation oil called AV1 that ended up destroying engines due in part to this problem (more on that here at BITOG and many other places). Mobil ended up buying ruined engines under warranty or lawsuit, then discontinued that oil.

That said, Aeroshell 15W50...

• 79Subaru

@2.7ecoboostFordBronco "Can you elaborate on what the additive issue was with Mobil AV-1? I might not understand your answer but I’m just curious. "

The answer is: The Mobil AV-1 product did not have the proper detergent/dispersant chemistry to lift and suspend the lead salts.

And with that we close this thread.