Non-ringed engines will take a "set", depending upon the fuel used. If you go from a high-nitro fuel, like your 30%, and then go to a low-nitro fuel, like 10% or 15%, then the engine may not respond well. Some will take some running time, others won't "come in" at all on the lower fuel. Usually, with non-ringed engines, you can go up in nitro, but not down. That's for large changes in percentage, and for percentages over 15%-20%. You can usually go back and forth between 5%-15% nitro fuels with no problems. It's not the oil, but the nitro content that's causing your engine's difficulties.

Ringed engines can usually take wider changes in nitro content, but you'll have to account for the need to change the compression ratio to suit the fuel.