ORIGINAL: kairos
One bottle I have left is Straight 20% castor 80 Methanol. I think it is the high castor oil that has done the damage.
It won't be the castor. I'm using 25% all castor in my ST G51 (there's a reason for that though) but all my other engines use 20% all castor. You're in England so most likely your castor is Castrol M which is what I use.