Engine oil weight/outdoor temperatures

I have been running 20W 50W Castrol in my 97 Villager because I remember a mechanic said it was better in reducing wear (long term), and would maintain lubrication better in the hot climate of Hemet, Ca. I now want to use something thinner, possibly synthetic for better gas mileage. I have 233,000 miles on the engine with this oil. Is it dangerous to switch to a synthetic now after using conventional that long? He also told me Walmarts' Supertech is just as good as Castrol?:runaround: