Sulphated Ash Vs TBN

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Feb 9, 2004
Just wondering, if you were looking at to oils and based on comparing the amounts of Sulphated Ash and TBN number, which is the better to look for? Eg: Higher TBN and with it the slightly Higher SA or Lower SA that brings with it a lower TBN or another way of looking at it is two oils are similar weight, rating and price, but one has say 10.5 TBN and 1.20 SA the other is 7.5 TBN and .75 SA (lets just say) What ones is better, and is there going to be much between them? Thanks!
I think two key factors in answering this question are quality and type of the base stock and hence, flashpoint. That's the "balance" in your two scenarios. Oils with a higher TBN will invariable have higher sulfated ash numbers, but if the flash point is high and the oil doesn't burn off, no problem. Oil that burns off more should have a lower sulphated ash number to reduce deposits, so it will have a lower tbn. I think base stock and volatility is the key. You need low volatility with a quality basestock to get high tbn.
So in looking at two oils that all else seems close, the higher TBN and SA would there for would most likely be the better choice?
Normally you want higher TBN, as long as the Sulfated Ash comes in below 1.5. But it is important that base stock not be volatile or it will flow through to the catlitic converter. SA is a factor of the additives. Calcium, Mag,zinc, phos, moly, etc. Normally the bulk of the TBN comes from the Calcium and Mag (detergent/dispersant) but there are also non-metalic dispersants that can be used (such as in two cycle oils) that are also blended into the regular oils to keep the SA low but the Detergency/dispercancy/TBN high. They are expensive. Some oils start out with very high Calcium/Mag levels and drop off like a cliff. Others maintain 80 to 90% of it over their usefull life. (I've seen Delo drop from 3200 ppm calcium to 3000 in 20,000 km, where Lubrax went from 3600 to 2000 in 20,000 km, while the Zinc in the Delo went from 1500 to 1300 and in the Lubrax went from 1300 to 850 with a similar drop in phos.)Quality of base oil and quality of additives are more critical than amount once they are above acceptable limits.
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