...from Mobil1's FAQ on their website:
"Can different synthetic motor oils be mixed together?
Mobil 1 is fully compatible with conventional motor oils, semi-synthetic motor oils and other synthetic motor oils, should it be necessary to mix them. However, it is important to note that the superior performance of Mobil 1 will be reduced by diluting it in any way. "
A quote from a Subaru enthusiast:
"At one time you could buy detergent free oil and detergent oil (as is the norm today), and additive packages that were "ashless" and some that were not. The ashless oils left less deposits in engines that typically burned several times the oil a modern engine will burn. Many years ago the additive packages did not play well with others and some of those oils would cause problems if mixed. Since auto manufactures have moved to standardized oil performance standards that is no longer a problem. Any oil that meets your manufactures basic service requirements can be mixed without harming the engine. You may see slight changes in extreme condition performance but nothing to worry about.
The oil suppliers and service stations fed this belief as a way to ensure customers kept coming back for service instead of buying oil where ever they found it at the best price. By and large all those issues disappeared in the late 1960's and early 1970's during the oil embargo when folks out of need bought what ever they could find, and discovered nothing bad happened."
Other oil manufacturer websites will tell you teh same thing; I've checked Amsoil, Shell, and Castrol to name a few. Here's a link for reference:
http://www.shell.ca/code/motoring/en...etic_oils.html
From that article:
"All grades of Shell oils, synthetic and conventional, can be mixed with no compatibility issues....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."
Do your own research and make an informed decision (as well as claims).
