Synthetic oils

I do mechanic jobs on the side and I recently did valve cover gaskets on a 1989 ford 150 van 302efi. When I took the valve covers off the motor was so clean I couldnt believe it.I asked the guy what oil do you use and he told me 10/40 walmart brand I think it was called TECH. He changes his oil every 3thou. If you are going to keep your car forever use synthetic if you are going to get a new one every 6 years use reg and save your money.