As you likely know, your pistol will chamber and shoot 38S&W ammo without any safety issues. It is marked 38-200 to denote the bullet diameter/weight of the original British cartridge. When international laws changed to outlaw the use of expanding bullets such as lead, the Brits went to a full metal jacket configuration which was much lighter but a bit faster.
The old original 38-200 round was very effective and with it's higher ballistic coefficients it rivaled the 45 caliber cartridges of the time.
About the only way to duplicate the original loading is to cast your own and hand load.
As for that "grease" clean it out. One hot day at the range and it will be seeping out all over your hands and travel box. Makes a heck of a mess.
Back in the day those were first released onto the markets, they were commonly found slathered in cosmo. I bought a box of six and all of them were wrapped in heavy brown grease paper with tags sticking out. Each wrapped pistol's outline only vaguely resembled a handgun and felt soft and mushy from all of the preservative grease inside. All of the pistols showed wear by the way. They had been issued, returned to stores, stamped for "sold out of service" marks, etc, floated in cosmo, wrapped and sent to International Firearms, then to Lever Arms. Great little revolvers and quite accurate. The recoil on them with the 200 grain bullets at just over 600fps was substantial in smaller hands.