Stuck rings can happen but it could be just worn out. I tired MMO and Seafoam, they did nothing. I fixed one with Kleenflo Combustion Chamber Cleaner, soak parts in this stuff for a few hours, it removes all the carbon and sludge. It happened after using our company's 15W40 diesel in my Olds 350. It started using more oil and a couple of cylinders were 25 to 50 low. Warmed it up, then fired it down the TBI till it stalled fired a bunch more down the TBI. I then pulled the plugs on the low cylinders and filled them and left overnight. Now it put out a hell of a smokescreen. It gained 20 to 35 psi. Between that and replacing oil seals on the valves, oil consumption was cut in less than half. Another Olds 350 had a choke staying on due to linkage issues, it washed out 2 cylinders, both 50 psi low. I did the the same thing, gained 15 to 25 psi. Unfortunately the cam was failing and took out the bearings. I actually bent the connecting rods, hydro locking the cylinders. If you can find it and try it, don't start with the cylinders full of fluid. Your neighbour's would complain of the smoke. Unfortunately, a good used 35 to 40 year old 3.8 motor is a crap shoot, if one can be found at all. A rebuilt one will cost into the thousands. Good luck,