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Is There Any Harm to Mixing Different Mobil 1 Viscosities?
 
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Question:
Is There Any Harm to Mixing Different Mobil 1 Viscosities?
I drive a '97 BMW 540i (85K.) on 0W-40, and a 2005 Subaru 2.5L (25K.) on 5W-30. I have used Mobil 1 for at least 10+ years now. I do keep drain intervals right at 5,000 miles on all of my cars, as it is easy to remember on the tach! Here are my questions: Is there any harm done if I were to mix and match between different viscosities within Mobil 1 products? I would prefer some thicker oil in our hot/dry summer months for both cars so can I simply mix in some 15W-50 with the base oils that I normally use, or is there really no need for that in modern engines? I just shiver at the thought of the relatively thin 5-10W-30/0W-40 oils all year round, especially when it hits 100°F+ around here, which can create tremendous thermal loads on any engine!
-- Konrad Werner, Albuquerque, NM
Answer:
Today’s modern vehicles require a less viscous oil in many cases than in the old days. Having said that there is no harm in mixing different viscosities of Mobil 1. They are all compatible with each other.  However, we think the 40 grade protection you get from the 0W-40 is more than adequate to protect your BMW even in New Mexico as is the 30 grade protection provided to your Subaru.
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