I also have become very interested in the 2 and 4 stroke issue. You most likely did not hurt your engine but do stop using it on the four stroke.
I have a 70 and a 91 FSII from OS as well as a 46,61 and 91 2 strokes. I have run quite a few other brands as well. The biggest difference is the oil but it doesn't stop with the total oil.
Some people along with glow fuel manufactures have various opinions about total oil 4 cycle vs 2 cycle. Here is the real deal.
Engines are subject to wear and more oil is better than less. 2 strokes are crude and they don't care. But the four strokes are more fussy. When it comes to four stokes ,
read the manual first. With OS they state that they can run 5% to 15% nitro with 18% oil and that oil may be 100% synthetic or a syn/caster blend typically 80/20.
I choose Synthetic 100% just because of the valve and gum potential alone.
Check out Wildcat 108 fuel 2 and 4 stroke from Tower hobbies.
Read the body of the 2 and 4 stroke fuel link from one of their fuel suppliers.
http://www.wildcatfuels.com/fuel_24c.htm
If you notice Tower will offer you both fuels for the same four stroke and they do not give you all of the details that you may want to know.
I buy Wild Cat because I trust what they say. Note that the fuel you can buy from Hobby Town USA is also made by them.
They tell you what's in the fuel with no BS.
You looked at your fuel and had to scratch your head about the mix of oil and if you ask most guys they will do the same. Some will say " I have been runnung this brand fuel for years and I don't have any problems. Hey ,people drink and drive for years too.
Final thought :
If you go with the 108 at 10% you only have to carry one gallon to the field.
Here are your options.
Tower sells the 15% nitro 100% SYN in quarts but not the 10% nitro 108 type.
http://www2.towerhobbies.com/cgi-bin...?&I=LXA300&P=0
Maybe your LHS will carry the gallon type at the best price because you don't have to buy quarts to avoid the hasmat shipping charge.
http://www.hobbytown.com/locator/
Good luck.
Eric