Ah, I see what you guys mean. So should I have really used the older AeroShell 500 in the Williams to begin with then? Or is it not so much that the newer formulations have more of a detergent action just that they are slightly different and any difference at all can cause problems?
Edit: To clarify what I mean, when I buy an engine they are usualy in need of a lot of TLC and all have the oil systems drained. So would it be best (if the manual doesn't make a clear recommendation so you can be fairly sure what had been used) to find the oldest generation oil that meets the specification in the manual, or instead find the newest. Or are you simply inviting problems whatever you choose
I'll guess I'll stick with the BPTO 2380 in the Williams as that's what it's been using since I got it, hopefully as it seems to be performing well there was no damage to the oil system. It will be interesting to see if it foams still after properly shaking the cans.
I'm a little bit worried about the GTD-350 now, I got the engine fully drained of oil with no paperwork so have no idea what was in it. The manual clearly states that AeroShell 500 is the manufacturers recomended oil so hopefully that's what they were using.