Just for fun here are some notes I had reason to put together a while ago. We may as well all share the wisdom they contain!
All mineral oils are produced from the same basic petroleum feedstocks. Because the base oil is molecularly the same, different oil viscosities will not separate or react negatively to each other. If you're concerned about the oil performing exactly how it was designed, what ought not to be mixed are the various oil additive systems used between brands.
Oil additives include:
Anti-oxidants to prevent thickening at high operating temperatures; Oour-point depressants which lower the temperature of wax coagulation; Alkaline materials to neutralize acids formed during combustion; Rust and corrosion inhibitors; Detergents to reduce sludge and varnish; Dispersant additives to hold contaminants in suspension; Extreme pressure additives to prevent metal to metal contact under high loads; Viscosity index improvers in formulating muilt-grade (viscosity) oils.
The exact type and amount of these additives varies from brand to brand and, when mixed, may not function as efficiently as desired. Different viscosities of the same brand oil will have different amounts of viscosity index improvers (polymers), but otherwise the additive systems will be the same.
Oil companies give conflicting advice on mixing products – we didn’t see that coming, did we!!
"Can I mix different viscosity grades of motor oils?" "Yes. It is always advisable to not mix motor oil brands, however, different viscosity grades of the same brand motor oil are compatible. Be aware that mixing viscosity grades will turn out a product that is different in viscosity from either what was originally in the engine or what was added."
From Shell: "If you mix viscosity grades such as a 5W30 low-viscosity oil and a 10W40 higher-viscosity oil, it is reasonable to expect that the resulting product will have viscosity characteristics which are thicker than the 5W30, but thinner than the 10W40. This change does not reflect incompatibility - it's simply a re-balancing of the viscosity characteristics. In all other ways, the product should work as expected. But there's absolutely no danger about incompatibility resulting from mixing engine oils,"
Miller says: "We've tested all of our grades and brands, and we haven't observed any problems."
From Mobil: "For our customers to choose a viscosity grade, we recommend they follow the engine manufacturer's recommendations as indicated in their owner's manual. There is no need to mix two Mobil 1 viscosity grades when one will do; however, we see no problem mixing different SAE grades of Mobil 1 Tri-Synthetic series motor oils."
