Mixing Marine Engine Oils: Compatiibility

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Mixing Different Marine Engine Oils: Reluctant Use and Compatability

http://papers.sae.org/961095/

Originally Posted By: precis
In engine maintenance there may be ocassions when due to some weighty reasons a change of the lubricant brand becomes nesessary. It is simply executed in small automotive type engines where a full oil charge can be poured off for the new one. However, in large marine engines containing much more oil charge which operate longer without change (many thousand hours) a similar procedure will be extremely expensive if the oil has not achieved it's quality(or performance) limit. For the most part it is done by gradual topping up one fresh oil to another oil in use. Are there any harmful effects on the engine and oil condition in that case?This paper presents experiments and test results utilizing different marine engine oils mixtures. The effect of some operational and external factors on the mixed oil properties as well as the study of oil incompatibility mechanisms have been investigated. Practical recommendations for technical service person have also been made.


I've only got access to the preview pages, haven't paid for it (yet, or ever, not sure yet).

Introduction introduces the "nothing blows up" principal of mixing fresh and fresh different oils.

Discussion on the nature of the dispersents used to handle soot loading and the introduction of an "incompatibility" ranking of soot dispersion, by basically developing a representation of the change in distribution of particle sizes when new oil with a different dispersent is introduced to a used oil...1 means no change, and "compatible"...numbers further away from 1 are the degree of incompatibility.

Table showing the TBN of virgin oils and those virgin oils mixed (interesting). And TBN drop in service of the mixes.

Lastly a few results and a little discussion on the differences between running a blend and topping up mid OCI with a different oil

YES it's old, is diesels, and marine diesels.
 
It's one of the reasons I bought a lifetime supply of the same oil. All the same batch too.

But then, I change my oil long before the contamination limit anyway, as it's my experience that going over just once can cost you money (immediately or shortly after). Much more money than changing the oil 10 times extra. It also allows me to fit the oil changes in to a much more relaxed timetable, doesn't have to happen tomoorw between 10 and 12 AM etc... next month will work aswell.

Mixing. I've done it to get rid of leftovers, I even had 4 different brands in the engine at the same time last year. But even if the 5w-30 isn't a 5w or SAE 30 anymore I'm so far from the limit (20W would work here in winter, and I shoot for 3.5 HTHS while the manufacturer wants 2.9 minimum) it's not going to make a meaningful difference.

But run closer to any limit (pumpability, HTHS, soot accumulation and dispersion etc) and I fully agree you're setting yourself up for failure or expense down the line. Which brings me back to the first line in this reply/contribution.
 
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