I am about to go to the hobby shop in about one hour. I don't know if I should get a .50GK 4-stroke or a .56GK 4-stroke engine for my new Hangar 9 Piper J-3 Cub. Money really isn't an issue and I don't know which I should get. I guess it is ultimately up to engine availiblility at the LHS. Also, what kind of fuel is burned in 4-Strokers? I'm a 2 stroke kinda guy and use 10% Nitro Glow fuel religiously. Will I be able to burn this same type and mix in my soon to be new Saito 4-stroke? Any help here would be great. If no one replies I'll just ask the guys at the shop, but they are too concerned about the dollar and not my plane, so there's no telling what they will recommend.

I have that plane.. I think the instruction say the .56 I found that even with a OS .91 4 stroke in mine I still needed 2 oz of weight to balance it. I'd get the heavier one.

I use the same fuel (cool power 10%-15%) in all my engines with no problems.

If you are stuck on these to engines, I'd say the 56 as it has more power. I am not sure if Saito still produces the 50. You'l probabaly need more weight up front. Move everything you can forward. The 56 will fly this airframe great. Some may sugguest a 72 which will give you the option of floats later down the road.
Use the 2 stroke fuel you have been using. 18-20% oil synthetic or synthetic/caster blend.

I am the proud new owner of a Saito .56GK 4-Stroke Engine. RC Hobbies charged me $198 for the engine and muffler. What a nice white and gold box

I will mount the engine as far forward as I can and I had them special order me a black heavy propeller hub to act as a weight of its own. I also bought tons of supplies I needed to construct the Cub as well as some nice 12x6 wood props. This plane is gonna look great!

Thanks for your help guys! I'll start a new post or something and keep a running log of this airplane's life.

You will like that cub

One modification I would recommend is using a sullivan tail wheel instead of the one included I found that off of a grass strip the wire enlarged the whole in the rudder.