Hi Top Gear PH,
Seeing the 10 pesos per liter promo of Phoenix because of their 10th anniversary surprised me and made me feel bad I did not make it. Though it made me think, ever since I have been fueling up my car with Petron’s Xtra Advance unleaded, will mixing other brands of fuel harm my vehicle? (I drive a 2013 Toyota Vios J)
Adding to my curiousness, will changing to other brands of fuel with the same octane level be better for my car or should I just stick with what I have been doing and continue fueling up with Xtra Advance?
I wouldn't feel bad about missing out on the Phoenix promo that you mentioned if I were you. If you don't regularly patronize the product anyway, why feel bad? I would probably have ill feelings only if I were a regular user and the particularly station that I patronize didn't say anything about it. That would give me second thoughts about sticking with the brand or finding another brand that's on par or better.
I think any car owner who's been driving for a while will have the same question in their mind as the one you asked.
The quick and easy answer is, any commercially available gasoline that passes standards will be safe to use with almost any gasoline-powered vehicle—usually. Save for certain vehicles which require the highest-octane fuel on the market, or those that require a lower-octane fuel because of incompatibility of the additive packages in the premium fuels with the respective engines' components.
Filling your tank with fuel from a different brand while you still have fuel from another brand shouldn't be an issue as well.
For your last question, if you're happy with the performance I don't see any need to switch. But if you've been wanting to try other brands to see which one works better for your car and driving habits, I’ll point you to a recent response I had to another reader asking about what's the best fuel for their car.
I'll also tell you that, for quite some time when I was driving a tuned car, I only filled up with the premium fuel that I had my car tuned with. I no longer drive that car. The gas I use now is the most affordable, most conveniently accessible, and delivers the best fuel economy that I can get with the car I drive now.