When i bought my outboard I was told by the retailer (and well respected marine motor mechanic) that instead of 50:1 mixture run 75:1. He said that the motor will last a lot longer, run smoother and will not carbon up. He said that Evinrude placed the 50:1 mixture on its motors as a precaution in case anyone ever had a drama with oil problems.
I recently took the plugs out of my motor for a change and put a camera scope into the bores to see how things were looking. After 12 yrs of running on 75:1 mixtures and countless hours the old 25 looks remarkably clean and there is no signs of scoring on the cylinder walls and no hot spots to be seen. The plugs are always clean looking and only need a quick wash in metho and replacing. I use surface gap plugs as the mech recommended and have had faultless service for possibly a thousand hours with them.
Oil I use is Shell Nautilus TCW3 usually. I did on an extended trip need to buy a 4L bottle of Mobil TCW3 and ran that through.
This old outboard is 12 yrs old now and showing signs of age with thew odd bit of corrosion around the yoke but it still is as good as the day i bought it as far as running goes. I hope my next motor will be as good.
Jack.