That is a lot of weight to add to an engine. I would be worried about the additional stress that you are adding to the motor and gears. I know that it is expensive, but it you want to pull long trains, you might be better off in the long run getting another engine.
Many years ago I added weight to my LGB moguls. I put about 6-8 oz of lead in the cab. They could pull longer trains with out slipping, but it wasn't long before I started having to replace the idler gears that connect the motor to the drivers. It took many years before the light came on. Since I removed the weights and run shorter trains, I haven't had any gear problems.
Do you have a traction problem? How many cars are you pulling when it starts to slip? How much grade are you pulling? Are you still using the traction wheels with the rubber tires and are the tires in good shape? I have an SD 70 and have changed out the traction wheels to metal wheels. So far I have towed 10 cars with it and not had any traction problems. I am going to increase it to 20 in increments of 5 and see if I still have traction. I will probably keep increasing the consist until it starts to slip. Then I will make the decision if I should have another engine or add weight. If I start to have traction problems I will probably start with 2 lb and increase it if needed. I plan on using shotgun shot and mixing it with epoxy and putting it in the belly tank on each end so as not to interfere with the speaker. I will make a bulkhead on each end to contain the slurry until it sets up. Hopefully i won't need it as I can see a potential domino effect with the four small screws that hold the belly tank on. I may have to use larger screws and mount some metal threaded nuts on the frame so they don't pull out. I do like my SD 70 it is a very handsome engine. Mine is in the BNSF orange/green std color scheme.
The locomotive is a Missouri Pacific/Union Pacific Heritage unit. The locomotive runs great and I have had no problems with it except trying to pull cars around 4 ft. radius corners. I have solved that problem and I posted how I did it. There are no traction tires on this unit. HURRY My railroad has 2% ruling grade and I have pulled maybe 12 cars up it with this engine, I like to run long trains. The thing is I have builted a track cleaner that works great and I can pull it with with my AristoCraft 2-8-8-2 (not Mallet I converted it to simple compound). It would be easer to hook it to the SD70 for that mid-day track cleaning after having run train all morning and the SD70 just can not do it without a little extra weight.
I picked 4 lbs. just because I had a 3 1/2 lbs. weight that I placed on top of the locomotive and would it then pull the cleaner plus have used that kind of weight in some of my steam locomotives and they have worked for years with no problems.
James, 4 pounds spread over 6 axles really does not sound like much... with the traction tires removed, then you have a safe way for wheel slip to occur when overloaded. I suspect the loads to your gears to be less than an unweighted loco when the traction tires "grab"... it makes sense.