I've been told by @Ozziedoc (who has been very helpful and seems well-informed) that for pot metal, in my case a Muhle R41 2011, silicone grease is the right solution. I read up on it a couple of days ago, and it seems like the correct answer for frail pot metal threads. (I treasure my R41.) Tomorrow I will receive 2oz of silicone grease, bought solely to keep my R41 in good working order.
Thanks pbrmhl. I saw this thread but decided not to respond lest people start thinking I'm a shill for silicone grease companies.