I have a Toyota yaris 2008 and use Vortex 98 in it. Can I use 95 even though when I refuel I always have half tank full then top up? Can I put 95 in the tank that already has 98 in it? Is the Yaris a good car?
Which fuel? Is a very interesting question.
The short answer is: You can use e10 or 91 in your car. It does not need 95 or 98. I just checked to make sure: the 2008 Yaris came with two different engines - a 1.3 and a 1.5. The minimum octane rating for both is 91. You are almost certainly wasting money if you put 95 or 98 in it.
On mixing the fuel: it does not matter if you mix 91, 95, 98 or e10 in any combination in your car. Don't worry about that. (In high performance cars that require 98 it is a mistake to put any of the lesser fuels in, and in cars that require 95, it is a mistake to put in any 91 or e10.)
Engines will always tolerate a higher than required octane rating, but a lower octane than the minimum required will damage the engine. The minimum octane rating for your car is 91. (E10 has a slightly higher octane rating than 91, but lower than 95.)
So - there you go. You can put whatever fuel you like in your car (except diesel).
Fuel companies often try to up-sell you to a premium fuel, but the benefits never stack up for ordinary cars. They always over-state the potential benefit of premium fuel. It's unjustifiably expensive for ordinary cars. In a Porsche, sure.
Often it is very confusing because of all the different names of the fuel: Vortex, V-Power, etc. What really matters is the number: 91, e10, 95 or 98. That's a kind of chemical standard they need to adhere to. The name is just marketing fluff.
Yaris is a good, reliable, cheap car. As long as you don't want excitement, you've come to the right place.