Hi stevej i too have a johno 70hp and have used the gulf western oil too, i know my motor was oiling up a little and had a funny smell to it as well..
i did find with the gulf western oil i had probs at low revs would just cut out all the time, so that is when i went from 100ml per 5 lt to 90-95ml of oil to 5 lts fuel and it was so much better, started better, run so much better and the smell was a lot less also
i have now gone back to the castrol oil now so will find out when the waether is better to go out...