Could different oils be mixed?

The issue regarding the miscibility of oils in most cases occurs when it becomes necessary to refill the oil. It is important to take into account the fact that not only base oils but also additives should be combined. Mineral base oils are mixed without restrictions. Semi-synthetic oils are well combined with mineral oils. Synthetic polyalphaolefin (PAO) oils also mix well with mineral oils. The compatibility of other synthetic oils (polyester, glycolic, silicone, etc.) with mineral oils and with each other depends on their structure. The requirement for motor and gear oils lies in their full compatibility.
Oil additives are thoroughly designed sets, so mixing may seriously disrupt their combination. It should also be taken into account that synthetic oils use other sets of additives than mineral oils do and this increases significantly the adverse interaction of all the components.
Oil manufacturers do not recommend adding another brand of oil, but if necessary, the following rules should be followed:

  • - Mix only oils of the same category, for example, API SG/CD oil should be added only with API SG/CD oil;
  • - Mix only oils made of same basic oils, for example, mix the mineral oil only with the mineral one;
  • - It is not recommended to mix synthetic oils with other oils.

For the same reason, oil manufacturers do not recommend adding any other auxiliaries or additives to oils.