At this point, I decided I should just buy the right bottom bracket. Easier said than done. FSA has no less than 6 BB's for MegaExo cranks. Their chart indicated I needed a BB-8681, but I couldn't find one in stock anywhere. I emailed FSA's customer support and here's their response:
I am sorry about the confusion over the BBs. I fully agree that it can be a little annoying… less complicated when you know the specifics but no less annoying when trying to look for a replacement.
First off, the BB-8681 is discontinued in the US. The 6200 has the same fitment as the 8681 and therefore works for alloy non-pinch bolt (NBD) as well as carbon MegaEXO cranks. This includes all Light series (SL-K Light and K-Force Light MegaEXO carbon cranks) as well as the newer aluminum Gossamer and Energy cranks that do not have a pinch bolt holding the left arm on. All of these require a wave spring washer on the non-drive side of the BB.
The BB-6000 is designed for the older pinch bolt style aluminum cranks. It is not cross compatible with the Light series carbon cranks as the spacing is too wide for them. Preload is done by the preload bolt that sandwiches the left arm against the BB. The light series and NBD cranks will overload this system and kill your BB.
I'm still not sure how the 6200 will fit, since I thought the carbon-armed cranks were narrower and needed the narrow bottom bracket. I guess it means that all their cranks are narrow now (without the pinch-bolts), so they all work the same. But it was nice to get a straightforward answer, and I thought I'd share it in case it helps someone else out.
