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.