Neatsfoot oil is usually only applied to leather products, ie piston washers as on say a W & S MKIII / Falcon / Jaguar or any gun with a leather piston washer. I have never come across anyone recommending neatsfoot oil on synthetic washers. Can I ask the question, why did you use neatsfoot on a known synthetic seal ? I have never known this use before as I'm sure lots of seasoned 'repairers' have never known it either. It is however an interesting reaction and I'm sure a warning to others; just glad you managed to recover the situation and restore the pistol, well done for that.
abellringer