RE: OS .25FP Any good???
Same "type" of fuel, yes, but not 29% oil, all of which is castor.
OS instructions for the FP series, dating from about 1991, state that fuel containing 20% castor oil is "correct." No mention of any synthetic oil %... (Further details: OS recommended 75% methanol, 20% castor oil, 5% nitro for initial break-in and/or general use, AND, later, after break-in, up to 65% methanol, 20% castor oil and 15% nitro for higher performance.)
With an iron piston and steel sleeve, you shouldn't need any more than the recommended 20% total oil. Even with 20% all-castor, that's a lot less mess than 29% castor.
Since OS recommends all-castor, you may find that ANY synthetic % - say an 18% castor, 2% synthetic oil portion - could prevent the castor varnish build-up that older types of iron-in-steel engines found to be a good thing. As wear occurred, that varnish layer seemed to maintain good seal and durable life. Synthetics are detergent, and can "wash off" the layer as it tried to form. However, the detergent effect may not harm things if the synthetic % is up to 5% (i.e., 15% castor, 5% synthetic.)
Newer (ABC/AAC/ABN) engines are manufactured to expand to "ideal" tolerances at running temps. A varnish coating would change the heating and cooling, and disrupt that intended fit at operating conditions.
Dennis is right that no FP was ABC, but I think the later ones were ABN - high silicon aluminum piston in a nickle-plated brass sleeve. Tower Hobbies had a near clone of the 40FP (many parts interchange) manufactured for a while. One main difference was that the Tower 40 was true ABC (hi-silicon Al piston running in a chromed brass sleeve. )