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"All grades of Shell oils, synthetic and conventional, can be mixed with no compatibility issues," he says. Miller is a Shell research engineer.
Shell has also tested compatibility of products with our four major motor oil competitors. While we can't always be sure of what changes they may be making to their formulations, we currently are not aware of any compatibility issues.
"It is certainly true that there are additive technologies and synthetics combinations available that could cause incompatibilities," Miller says. "But Shell does not use them, and I know that if that ever changed, we would be sure to keep everyone very well informed."
At the same time, it's important to understand just what constitutes incompatibility.
"Incompatibility of engine oils refers to observations such as additive dropout, or changes in fundamental oil performance that's not what one might expect from mixing two different oils together," Miller says. "But naturally, if you mix oils with different characteristics, you're actually blending those characteristics - you've changed some of the features of the oil."
For instance, 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.
"Using our Shell products as an example, if a car specifies a 5W30 oil as the preferred viscosity grade, then FormulaShell 100% Synthetic 5W30 provides the best performance we have," Miller says.
"And if you find you need to top up with another Formula Shell there will be no compatibility issues, but the mixed product's performance characteristics will not be the same as if only FormulaShell 100% Synthetic had been used.
"But there's absolutely no danger about incompatibility resulting from mixing Shell engine oils," Miller says. "We've tested all of our grades and brands, and we haven't observed any problems."