I recently purchased a 4 stroke engine and I too do not want to buy 2 separate types of fuel. I did some research on this and found out that it is pretty critical to the life of a 4 stroke engine to have caster in the fuel. I believe someone else mentioned that synthetic doesn't stand up in the extra heat of a 4 stroke. So, what I finally came up with is this....get the 2 stroke fuel of your choice and add a small amount of caster oil to it. I have been doing this in a smaller bottle. I add 1/4 oz caster to 10 oz fuel. This should bump the oil content up by 2%. So you can buy the fuels with less oil in them such as 18% and bump it to 20%.
good luck to all Jeff