What petrol?

We have an old Mariner Marathon 10 hp outboard, serial no. OG007024. Hasn't been used in years, wondering if anyone can tell me if it uses outboard fuel or Unleaded? Thanks Terri

Regular Unleaded mixed 50:1 with quality 2-stroke oil.
For an engine that has sat for awhile... the first thing you want to do before starting is a compression test. This could even be as simple as placing your finger over the spark plug hole- and cranking(or having someone else pull the rope) it over. If it blows your finger away.. you have "enough" compression. If not, I'd search for a cause before starting.
If you have compression, next check the spark. If it's a nice blue spark that will jump a 1/4" gap... you should be fine to try and fire it up.

I prefer to install brand new spark plugs (not just clean what is there) , and a mixture of 40:1 for the first couple tankfulls.

Once it is running- check to see if the waterpump is pumping water- if it is... you are safe to run it awhile to get it warmed up etc.. If not, shut it down before it get's hot enough that you can't place your hand on the head... and have the impeller replaced.

Thanks so much for your help! Great to see someone knows what they are talking about! Have a great day! regards Terri