I gave my son similar advice when he bought his first car with a Mach 28 engine. I told him to just use his plane fuel which is 80/20 all castor and it ran like a charm . As for what the shop owner told you, I wonder if he knows that ABC control line speed engines (which are far more powerful than car engines) use exactly the same fuel as we used in the car?

No engine needs nitro, even one with very low compression.