Leading in barrels is usually not that much an issue. There are some bullets and loads that will certainly clog up the rifling. But the better bullets are cast with alloys that won't really cause leading issues. 100 pounds of lead and 13 pounds of tin is a pretty good one, anything close to that should be okay. I for one, don't have a clue what the big factories use for slug alloys. You may be better off by finding a commercial reloading company and using their loads for your gun. Look for a large public range that shoots a lot of 38 special, and has in house reloads to buy. You want to find out where those reloads come from and buy direct. If you can spring for a thousand at a time, or more, often those reloading companies will be happy to load what ever you wish for slugs and powder.
Low velocity is no guarantee of no leading. On the other end of the spectrum, the Keith bullets, properly cast, lubed, and loaded at magnum velocities, are known as bullets that don't cause leading issues. There are other lead slugs that shoot cleanly too. All said and done, you can make a habit of firing off a few hundred lead slugs and then fire a couple of full cylinders of jacketed bullets-that will scrape out the lead quite well at the end of a session. When you clean the bore, use something good like a CLP, and let is soak for a few hours before using the bore brush.