A lot of oil is fine, however I would suggest you consider using simple volumes for measurement and not specific gravity. Nitromethane is very dense and adding 10% by specific gravity leaves you short of nitromethane volume, that is you do not have 10% actual volume in your fuel, it is quite a bit less, based on the specific gravity of nitromethane. With many engines, this lower nitro is okay. If oils are not very dense, and they are certainly nowhere near that of nitromethane, you might take a look at the actual VOLUMES of each components you are using. One of the very simplest ways for folks to do homebrew on a small to gallon size lot of fuel is to either use 1000 ml Pyrex measuring cups (a little over a quart) or, better yet, a 4000 ml (glass or Nalgene) graduate cylinder. Percents are EASY using metric. For example, if you wanted 10% nitro in 4000 ml of fuel, then add 400 ml (.10 x 4000 ml.)to that 4000 ml cylinder. If you wanted 18% final oil, then add .18 x 4000 ml of oil and so forth. Some fuel manufacturers are using weight instead of volume and their 15% nitro has a LOT less than that in terms of simple total volume of the fuel. In my view, this is very deceptive and confusing to modelers to do this.