Tower doesn't know. They 'believe' it's a castor/synthetic blend, but have no idea what the percentages would be. Actually, a little research showed that Tower brand does not have ANY castor oil. To get a castor blend from Tower, you must buy a 'premium' fuel, such as Wildcat brand. And, it's important to note that only the Wildcat Extra Premium has 18% lubrication with castor; the regular Wildcat premium is 15% to 16% lubrication, which is lower than most engines seem to recommend.

The OS manual says anything goes, as long as it is 18% lubrication (5-15% nitro, and synthetic or blend is fine).