DON'T DO IT!!! All an engine flush is going to do is possibly knock loose debris and clog an oil passage and then your screwed.
I recommend a high quality Diesel oil for a couple of reasons. First, the Diesel oils tend to still have higher quantities of zinc, which is critical to our flat-tappet engines. Second, they have higher levels of detergents, to keep our engines running cleaner. Thirdly, their dirt cheap and can be sourced almost anywhere. I personally run Valvoline Premium Blue 15w40 year round. They also have a synthetic if you want to go that way.
BTW-It's a myth that you have to flush an engine to swtitch to synthetic.