Concerns of Mixing Oil

• When synthetic oils first came onto the consumer market, the synthetic oil reacted differently with engine gaskets and caused them to shrink or expand and often resulted in oil leakage. Those problems have been resolved, and you can now safely switch between conventional and synthetic oils.
  
  

Mixing Castrol Synthetic Oil

• According to statements on the Castrol website, you can safely mix Castrol synthetic oil with other conventional and synthetic motor oils. Castrol even produces a number of synthetic oil blends that it sells to the public.
  
  

Reasons to Mix Oil

• Mixing conventional oil with Castrol synthetic oil provides a way to gain some of the advantages of synthetic oil while saving money compared with the cost of using full synthetic oil. You can also safely mix Castrol synthetic oil with other brands of oil when adding oil between changes.