First, try finding 98 octane at the corner gas station. It might be a long search, but the usual pump gas generally is just fine for all of the engines typically used for r/c.

If you want fuel without methanol/ethanol additives you will have to look for aviation fuels (not recommended) or racing fuels. C-10 is a 100 octane racing fuel that contains zero methanol/ethanol additives. It's not cheap!

Typically there's no issue in running a higher octane fuel in anything. It just costs more than needed when used in an engine of lower compression. High octane fuels will not change the carbon development in your engine. The oil type, ratio, tuning, cooling, and muffler will be the controlling factors. Using more oil containing detergents to rinse cylinders goes far in reducing carbon development and extending engine life. Racing oils typically have lower detergent levels. Engines using more oil have been proven in both the lab and the field to last 3 to 4 times longer than engines using less oil.

Considering that for 2 bucks or so you can obtain a bottle of Stihl or Echo 2 stroke oil just about anywhere, wouldn't that be easier than fretting about the old oil? Both work great up to about 50-1 and will get you flying today.