P0300 is a Cylinder Misfire Detected Random Cylinders error. By itself, doesn't mean much.
How immediate is "immediately"? As soon as you started the engine? If so, then it may not be the gas as the new gas takes a while to make it from the tank to the engine.
The easy things to check are:
* Battery condition - a weak battery can cause problems, especially with modern, computer-controlled engines.
* Water/dirt in fuel - try some fuel system cleaner with methanol to clear the water. May need to change fuel filter.
Not so easy:
* Dirty TBs - after 61k miles, they probably need a good cleaning. DIYs on this site. If you decide to clean the TBs, go ahead and clean the MAFs while you are there.
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