Here is likely more than you will ever want to know about real world motor oil facts from a fellow motor head / test engineer , 540 RAT , the only oil he rated as " out standing " in 20W50 was Castrol GTX conventional oil @ 96,514 PSI , thermo breakdown occurred at 275deg

https://540ratblog.wordpress.com/201...-test-ranking/

 

Reviving the thread!

I'm not an oil expert but I have been doing a lot of reading about oil as of late. Here is my two cents. I would appreciate if an expert could chime here and indicate if what I'm saying makes sense.

It is all about bearing clearance and oil pressure. An oil that is not viscous enough (I avoid the word "thin" as I had read that viscosity is not a spatial concept but a flow vs oil pressure one?) will not offer adequate film strength to keep the bearings from coming in contact with the shaft. Harley have loose bearing to shaft clearances by design (if someone can give comparative tolerances between a Harley engine and say, a tight car engine, I would appreciate it!) A 10w40 oil does not have the required viscosity @ 100 C. Whether the viscosity @ 40 C of a 10W40 is acceptable, I'm still not sure. On the one hand, a lighter weight oil will lubricate faster but will it have the required film strength for those looser Harley bearing to shaft specs. The is important especially at start up. This is why IMHO I would certainly not use Castrol 5w50 in a Harley engine. The cold weight would cause premature wear of crank and cam bearings. It should be ok at operating temps but the amount of VI improvers in this oil must be high enough to bring it to a 50 weight. I think these improvers should be avoided as much as possible so the less the spread between the oil grade numbers, the better - to a certain point! I'm still looking for an answer to that question. Please do not quote the manual, we all know what it says. I want would like to know what the lowest viscosity @ 40 C can be safely used in a Harley engine. It is also a known fact that the thinner (I said it! lol) the oil, the less friction it causes and the less heat it produces. We all know that heat is a trouble maker in an air cooled engine. Another factor to keep in my is thermal breakdown. An oil has to be able to resit the high temps the Harley engine dishes out for it's entire service interval to be effective at protecting all the engine bits.

There is also the question of oil pressure. viscosity= resistance to flow. oil pressure is dependent upon viscosity. If the oil is not viscous enough, the oil pump will not be able to effectively create the required oil pressure to pump the oil adequately to all the engine bits that require it.

Some literature I highly suggest we all read:
http://motorcycleinfo.calsci.com/Oil.html
https://www.enginebuildermag.com/201...ng-clearances/

So that's it. Can a guru step up and clarify all of this for us please?
Thanks

 

Interesting read. I admit, I've skimmed over the testimonials. It's a long read! What I found interesting is the relationship between viscosity and oil pressure. He states that oil pressure is directly related to viscosity but to use the thinnest oil possible. He also suggested a more efficient oil pump to pump thin oil. Thin oil=less heat and faster oil travel time through the engine. This means that a thinner oil will keep engine temps down, a favourable situation for an air cooled engine.

So, if I go with his logic, I should replace the existing stock oil pump with a high performance one, say a Feuling or S&S oil pump and use a thinner oil. This should maintain adequate oil pressure while reducing engine heat.

Has anyone with an upgraded oil pump and oil pressure gauge ever tried a lower viscosity oil in their Harley engine to determine what the results are? I'd sure like to know what the results would be. My one concern with thinner oils is whether the cam bearings are adequately protected.

 

Interesting read. I admit, I've skimmed over the testimonials and haven't read it all yet. It's a long read! What I found interesting is the relationship between viscosity and oil pressure. He states that oil pressure is directly related to viscosity but to use the thinnest oil possible. He also suggested a more efficient oil pump to pump thin oil. Thin oil=less heat and faster oil travel time through the engine. This means that a thinner oil will keep engine temps down, a favourable situation for an air cooled engine.

So, if I go with his logic, I should replace the existing stock oil pump with a high performance one, say a Feuling or S&S oil pump and use a thinner oil. This should maintain adequate oil pressure while reducing engine heat.

Has anyone with an upgraded oil pump and oil pressure gauge ever tried a lower viscosity oil in their Harley engine to determine what the results oil pressure cold and hot and the temperature of the oil itself? I'd sure like to know what the results would be. My one concern with thinner oils is whether the cam bearings are adequately protected. We must remember that bearing to crank tolerances in a racing engine are much tighter than the looser Harley engines.

In the article, the author states: "The average value for Thermal Breakdown among the conventional Diesel oils here, was 255*, which is 17* LOWER than among the conventional gas engine oils I’ve tested. The average value for Thermal Breakdown among the synthetic Diesel oils here, was 267*, which is 15* LOWER than among the synthetic gas engine oils I’ve tested." If thermal breakdown is important in an air cooled engine such as in a Harley, why does Harley recommend using diesel oil instead of HD oil if HD oil is not available?

 

The level of detergents and other additives is generally higher for diesel motor oils, to combat the higher soot levels of diesel engines.
Higher additive levels in diesel oil means you can drive further before they degrade to levels too low for a gas engine.

Diesel engines will have higher combustion chamber temps because of the higher compression ratio.
It takes a temp of ~ 600 F to ignite diesel fuel. (gasoline has an auto ignition temp of about 495 F, E10 about 525 F)
Diesel fuel in general also has a higher Hydrogen content, that will tend to produce more water when it burns.