I used a high dollar gun grease (Pro-Gold) for many years but in the last several years and lots of reading, I have decided that grease of any kind is not good in a lock. The reasoning is that grease will more readily catch and hold fouling. While an oil will catch fouling, if it is applied lightly, the amount of fouling it will catch will be minimal. I am of the opinion that a lock should be cleaned every time the gun is cleaned. As long as the lock is kept clean, any good machine oil such even 3 in 1 will do a fine job. Break Free CLP is another good one. Just apply it sparingly to the working parts and a very light film on the non-working parts will keep it looking and working good.
Another product I am trying is Frog Lube. It is somewhere between a grease and an oil in consistency. It has worked wonderfully on my 1911 and other modern weapons and is showing promise as a lubricant for my muzzleloaders. But that is still in the trial stage but looking really good. I would not use Frog Lube in my muzzleloading bore simply because so far it is an outstanding lubricant and will make your bore too slick. Being too slick will make your bullet or patched ball start to move forward before the powder has built up a good breach pressure. The result is variation in some muzzle velocities. My present preference is to use the Frog Lube on the exterior of my rifles and as a lubricant on moving parts.
I have tried dry graphite in the working parts of the lock on one of my rifles and I am just not convinced that it is a good lubricant for a lock. The jury is out on that one, too.