If you look at page 1.5 of a Kohler Service manual tells the owner it's ok to use clean, fresh 87 octane or better unleaded gasoline. Using leaded gasoline will leave more carbon deposits in the combustion chamber and the cylinder head will need more frequent service. ALL small motors reguardless of their age were design to run on unleaded fuel from the factory.
The section aslo says it's ok to use up to 10% alcohol in your Kohler. Gasohol (term for E10 gas in the 1980's) came on the market in the early 1980's. You won't have any problems using E10 if you have a good fuel/storage program. I would use a fuel stablizer in all your small motor fuel (2 and 4 cycle). Sta-Bil and one that Briggs and Stratton sells are 2 good ones to use.
Higher compression motors need higher octane fuel. A motor should have 9.25/9.5 to 1. before it need higher octane fuel. Kohler motors from the factory have 7 to 1 compression ratio. Milling the head and decking the block may raise the compression to 8/8.5 to 1 maybe 9 to 1....not enough to use high octane fuel.
When I go to different plow days I like to try a few things. One was trying 91 octane fuel. The performace level on my 149 was worse using 91 octane fuel. My 149 has .040 off the head and .005 to .008 off the top of the block.
All gas will go bad. Some just take longer.
Briggs and Stratton has an inline fuel filter that works good on my Cubs.
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