A nagging question about oil Flash Point?

S

Slippery Customer

Joined
Jul 14, 2026
Messages
8
As a long time viewer of this site, I must say it is a tremendous resource for all things oil. So firstly a thank you.

I decided to post for the first time as I have one nagging question that I would be grateful for your thoughts…
Over the past several years I have noticed that oil ‘Flash Point, ºC, ASTM D92’ seem to be falling. I’ll use Mobil 1 ESP 5W-30
as an example. The figures below are taken from the manufactures own data sheets.

Year: Flash Point, ºC, ASTM D92
2005: 254ºC
2018: 239ºC (not LL04)
2026: 221ºC

I don’t know if this is a general trend or something specific to Mobil 1. I just happen to have old oil spec sheets for this oil, as I mostly use
Mobil 1 ESP 5W-30 (LL04) in my 5 Series diesel BMW. I asked Mobil 1 tech support about this a few years back and didn’t get an informative answer.

So my questions are:
How important is the flash point to overall oil quality?
Why is the flash point dropping like this?
 
Thank you, I broadly understand the test. Perhaps I wasn't clear, I was hoping for answers to two specific questions:
How important is the flash point to overall oil quality? Why is the flash point dropping like this (example provided)?
 
Last edited:
I have been lead to believe that a higher flash point means higher percentage of synthetic groups.

I have also been lead to believe that oil specs are more like Johnny Depp guidelines and the brew can change long before the spec sheet.
 
More volatiles (cheaper) in the VII mix. Common for lighter weight base oils these days too, mainly for the thin crowd 😁.

Flash point is an important component of an overall oil quality, especially with evaporative losses. But good formulators make it all work even with a lower ASTM D92 numbers.

Fuel dilution is way more detrimental to the flash point than whats in the oil itself.
 
Slippery Customer said:
As a long time viewer of this site, I must say it is a tremendous resource for all things oil. So firstly a thank you.

I decided to post for the first time as I have one nagging question that I would be grateful for your thoughts…
Over the past several years I have noticed that oil ‘Flash Point, ºC, ASTM D92’ seem to be falling. I’ll use Mobil 1 ESP 5W-30
as an example. The figures below are taken from the manufactures own data sheets.

Year: Flash Point, ºC, ASTM D92
2005: 254ºC
2018: 239ºC (not LL04)
2026: 221ºC

I don’t know if this is a general trend or something specific to Mobil 1. I just happen to have old oil spec sheets for this oil, as I mostly use
Mobil 1 ESP 5W-30 (LL04) in my 5 Series diesel BMW. I asked Mobil 1 tech support about this a few years back and didn’t get an informative answer.

So my questions are:
How important is the flash point to overall oil quality?
Why is the flash point dropping like this?
Click to expand...
That trend matches what appears to be a trend in less PAO in the formulation, at least with the 0W20, to which I have paid more attention.
 
Thanks Shortyb and Indian. Yes, I wondered if the change in flash point reflected changes in base stock formulation towards group III.

I take your point about fuel dilution affecting flash point. Would the flash point simply reflect the most volatile components, such as the fuel dilution, or the average volatility of the mixture?
 
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Slippery Customer said:
Thanks Shortyb and Indian. Yes, I wondered if the change in flash point reflected changes in base stock formulation towards group III.

I take your point about fuel dilution affecting flash point. Would it be correct then that an oil with a higher flash point would cope better due to the higher starting point?
Click to expand...
Cope how? With igniting?

What are you attempting to achieve? Use flash point as a proxy for oil quality? IMO it is one of the least important properties listed on an SDS, and is there for fire fighters and first responders. It's also one of the least precise tests.
 
kschachn said:
Cope how? With igniting?

What are you attempting to achieve? Use flash point as a proxy for oil quality? IMO it is one of the least important properties listed on an SDS, and is there for fire fighters and first responders. It's also one of the least precise tests.
Click to expand...

As stated, I am trying to clarify how important flash point is to overall oil quality (because it seems to be falling). You say not. Thank you for that. Care to elaborate? I have also rephrased my question about fuel dilution.
 
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Slippery Customer said:
As stated, I am trying to clarify how important flash point is to overall oil quality. You say not. Thank you for that. Care to elaborate?
Click to expand...
My understanding: It's not a test of quality, it's a QC test to confirm the batch was blended properly, it establishes a baseline for future testing for fuel dilution, it establishes what safety protocols are required for flammable liquids.
 
I think what Slippery is getting at with flash, in regard to "quality", is possibly how the oil holds up, not about how volatile it is per se. I could be off base and he is a closet firefighter type that may want to know how likely it will be to battle a garage fire though 😁.

Flash point is a multi faceted measurement, and probably most useful for how likely an oil withstands heat and breakdown from a thermal standpoint (oxidation). Having a higher flashpoint does not necessarily equate to it being able to better cope with fuel dilution, but
does provide some hedge. There are other constituents in the blend to better help maintain oil film strength and prevent viscosity drop that are not part of the flash point's volatility measurement. Some of these are moly and boron to help strengthen the oil against thinning by fuel.
 
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kschachn said:
It's also one of the least precise tests.
Click to expand...
This. Especially with used oil. GC (gas chromatograph) is probably the most accurate testing method for flashpoint. The "cheaper" testing labs don't use it or use it improperly to interpret an accurate flash point result. I won't use the Blackstone word. Oops 😁.
 
Leo99 said:
My understanding: It's not a test of quality, it's a QC test to confirm the batch was blended properly, it establishes a baseline for future testing for fuel dilution, it establishes what safety protocols are required for flammable liquids.
Click to expand...
Thanks. I am finding conflicting details on the web and hope I can find clarity here. Mobil for some reason don't quote Noack volatility
on their Euro data sheets; they only quote the ASTM D92 Flash point. Hence it is used for the comparison above. Most of the other parameters for Mobil 1 5w-30 ESP don't change that much from 2005 -2026. (A possible exception being sulphated Ash, wt%,ASTM D 874 that went up to 0.8 from 0.6 in 2018).
 
shortyb said:
This. Especially with used oil. GC (gas chromatograph) is probably the most accurate testing method for flashpoint. The "cheaper" testing labs don't use it or use it improperly to interpret an accurate flash point result. I won't use the Blackstone word. Oops 😁.
Click to expand...
Yeah I used to perform flash point tests, with automated equipment it is more repeatable but the repeatability and reproducabilty is still not good for the test.

No standardized test uses it for some general quality parameter. If someone wants to gauge the quality of a Group III base, that comes from saturation, contaminates and viscosity index. You’d be better off making a correlation to pour point, but even there the presence of pour point depressants can make a significant difference.
 
shortyb said:
I think what Slippery is getting at with flash, in regard to "quality", is possibly how the oil holds up, not about how volatile it is per se. I could be off base and he is a closet firefighter type that may want to know how likely it will be to battle a garage fire though 😁.

Flash point is a multi faceted measurement, and probably most useful for how likely an oil withstands heat and breakdown from a thermal standpoint. Having a higher flashpoint does not necessarily equate to it being able to better cope with fuel dilution, but
does provide some hedge. There are other constituents in the blend to better help maintain oil film strength and prevent viscosity drop that are not part of the flash point's volatility measurement. Some of these are moly and boron to help strengthen the oil against thinning by fuel.
Click to expand...
Thanks shortyb that's very useful. I am not a firefighter but it is a noble profession. Mobil only report ASTM D92 Flash point in the data sheets I have. Since it has been falling from 2005, it was used for this comparison. I accept that Noack volatility would probably be a better measure of volatility but it's not presented. I am just trying to understand the implications for the oil given the nature of the ASTM D92 measurement, and why the flash point is falling.
 
Leo99 said:
My understanding: It's not a test of quality, it's a QC test to confirm the batch was blended properly, it establishes a baseline for future testing for fuel dilution, it establishes what safety protocols are required for flammable liquids.
Click to expand...
But flashpoint testing IRT fuel dilution is supposed to be strictly a screening test not a baseline determination. My biggest beef with those labs that use it as reportable values on a UOA.
 
 
https://bobistheoilguy.com/forums/threads/flash-point-base-oil-quality-performance.292593/

Tom NJ, great, this is just what I have been looking for!

Sorry to persist but you mentioned "The problem with flash point is the test reproducibility by ASTM D 92 is +- 9°C (+- 16°F). " In the case of the three generations of Mobil 1 5w-30 ESP above, the difference reported in flash point between 2005 and 2018 is 15°C and 2005-2026 is 33°C respectively: i.e. well outside that sampling error. Care to comment about what might be going on?
 
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