My 2005 XF is detonating!

I don't drive it much, and it did fine on the tank of gas from/during the winter. But then I filled it with the local Premium gas (only 91 octane here) a couple of weeks ago, and soon got detonation.

I've been reading the related threads here, and elsewhere. Sounds like at least two things: 1) warmer/less dense air, and 2) low octane.

Adding a jar of lower-cost octane booster only slightly reduced the problem.

After reading a thread here, I'll now add a bottle of gas-dry and check the error codes (no warning light showing now).

Could have been bad gas from the BP station. Insert BP jokes here ...

Other ideas?