TBN more important than viscosity?

[David_g]

A lot of times I see a used oil analysis from a shared sump bike where the oil has sheared out of grade but the TBN is still good. And some will suggest running the oil longer.

As I understand, when TBN drops to a certain point corrosive wear can occur.

But I thought when oil shears out of grade that galling can occur.

Is shearing from 40 weight to 30 weight not likely to be a problem? Is it OK to ignore the oil losing a grade when TBN is good? It would give us longer OCIs...

 

[tc1446]

David, I don't have an answer but am interested also. Out of curiosity, what numbers are you thinking might be good for TBN? I've had a couple tests done and with the viscosity sheared down to 30 range, and TBN at 6.6, it was flagged "Caution". Diff oil, similar viscosity, but TBN at 9 was given "green flag". Both these were done at 3000 miles...not much on synthetic oil. Nevertheless, I've settled on oil changes at 4000 mi using Amsoil MC 10-40.

 

[Pablo]

Quote:
TBN more important than viscosity?


In general: NO

At the extremes, viscosity is WAY more important. Say you need 10cSt. You try and use an oil that is 0.0001 cSt or an oil that is 1,000,000 cSt you will have problems immediately.

There was a used oil analysis with TBN of 0. The engine was doing OK - not advisable at all, but just won't build up immediately.

 

[cat843]

TBN is how well the additives are still doing. Viscosity along with oxidation and nitration are how well the base oil is doing.

 

[Cujet]

No way. Oil viscosity is the key. There are oils that have no additives to combat acids, such as straight 30. Yet, these oils do a very good job of lubricating engines, when used properly.

 

[Gabe]

Originally Posted By: Cujet
There are oils that have no additives to combat acids, such as straight 30. Yet, these oils do a very good job of lubricating engines, when used properly.


Non-detergent oils should not be used in a motor.

 

[BigCahuna]

Look at the past used oil analysis or even the voa's. I've never seen a comment that the oil is too old to be used. Might say it's dropped a few grades, and that doesn't seem to be a problem. The mfgr want's you to buy more oil, so I can see their view on it. Age doesn't seem to bother Blackstone, unless the oil is from the 1940's.,,

 

[Qwiky]

Here is a good read on the subject.

http://www.machinerylubrication.com/Read/2170/oil-drain-interval-tan-tbn

TAN is not always meaningful indicator as some acid types may be more corrosive at a lower concentration than another acid type.

TBN depletion is generally considered to be reached at 50% of its virgin level.

 

[Ken2]

I'd assume that the engine designers plan on the shearing and specify an oil that will protect the engine and transmission for the full recommended oil drain interval despite the shearing.

A member on the V-Strom forum has 276,000 miles on his DL1000, no engine or transmission repairs, and has always run Castrol Syntec (now Edge) Full Synthetic 10W-40 automotive engine oil with 6,000 mile oil drain intervals. 3,333 miles is the recommended interval. Shared engine/clutch/transmission sump. He uses Fram filters.

 

[Qwiky]

Holy smoke, 276k miles.