Hi, Robin - I'll go along with Mark on this one and add something from my personal experience.
My first FP .40 did indeed have the runaway problem. I was told that adding head shims would help but I also considered minor head modifications.
Since I had a spare FP .40 head, I decided to experiment with reshaping the combustion chamber mainly by enlarging the existing hemispherical chamber and reducing the width of the squish band. This would, of course, lower the compression ratio considerably.
On the particular engine I had, a radius of .48 inch and a .050 wide squish band worked best. The runs were perfect, yielding a very nice 2-4 break and it never ran away. I was using Powermaster 10/22 fuel and an 11-6 Rev Up prop. I tried a number of different plugs and they all worked well. An OS ratcheting needle valve did the trick.
A friend of mine was having similar problems with two FP 40s and he asked me to modify them. At the time, it was popular to reduce the blowdown by lowering the ports (intake or boost, I do not recall) so we decided to try the head mod on one and the port mod on the other. After many test flights, we decided that although both mods worked, the head mod yielded the better results.
My personal preference is really the head mod. When cutting a plated sleeve (AAC, ABC, AAN, ABN, whatever) you are grinding though a plating interface. Doing so can allow the plating to peel away at some future time, with disastrous results. The head mod on the other hand is quite safe. If you're not pleased with the results you simply replace the head with a new one. If the sleeve mod is not satisfactory, you must replace piston and sleeve ($$$).
I have performed this mod on many engines including the K&B 40, FP 35, LA 40, Bodak 25, Tower 40 and ST 60. I did them for people at no charge and all feedback was very positive. I tried it on the LA 46 but it made no noticeable difference - the engine ran perfectly before and after.
Regarding the addition of more oil, I had a mild runaway issue with a Fox 35. Since I did not have the opportunity to modify the head at the time, I tried extra shims -they did not help. I added castor to the fuel to bring the total content to around 25%, left the head shims in and the runs were right on - a very nice 2-4 break.
As I always state, this worked for me - individual results may vary.
Bob Z.