Depends on what's in your crankcase and how you drive it.
I use Mobile 0W40 fully synthetic in the winter and Castrol TWS 10W60 in the summer. Get better gas mileage with the Mobile and less shear during cold start ups in the winter.
I use it as a replacement oil--don't like to have a potpourri of oils in the crankcase. However, that said, you should be fine.
If you already have a Mobil product in your vehicle, then yes, feel free to top off with it. Alternatively, feel free to change over to it at a later date.
As far as topping off goes, priority (assuming a synthetic in the crankcase) is:
1. A synthetic of a different weight from the same manufacturer,
2. A Dino oil from the same manufacturer,
3. A synthetic from a different manufacturer.
Why same brand dino over different brand synthetic? Additive packages and possible incompatabilities.
What are (some of) you people talking about?!?! It is never OK to mix oils, period. Unless you KNOW, for a fact, that they are compatible. One oil in a 5-30 may be entirely different than a 10-60 or even a 5-40...same mfg, etc
Doug has it right above^^^. Additive packages may be very different- nobody really know what is in these either. (Unless you live over on BITOG)
You 'top off' with the oil you put in when you changed it. If you don't know what is in it, then you really DO need to change it.
Of course, in an emergency, it is OK to add ANYTHING. Just change it ASAP.
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