Yes that helps. One of the engines is a V-6 and one is a V-8, so your answer cleared that up for me. Thanks !
Your engine is mostly aluminum and the engine block actually is 2 halves connected together. I have worked on literally dozens of these engines, and I must say, just about every one out there leaks a little oil to some degree.
Your quote of some 3000 to repair, was most likely for an engine removal and reseal. Most of the money would be for labor as this engine is very labor intensive.
To remove the engine, completely, dis-assembly, reseal, and then re-assemble, would be around the 20 to 25 hr labor estimate. Depending on the shop's labor rate, with parts, it wouldn't take too long to spend your 3000.
Another issue that has to be considered here, upon building this engine at the factory, threads are tapped into aluminum. The bolts have "lock-tite" on them, and when one of these engines are dis-assembled, many times the aluminum threads are pulled out. Now we have to drill, re-tap the hole, and install a steel piece called a "time-sert". This too is labor intensive.
You ask why is this leaking and why so much. Well, most of the seals on these engines are "spaghetti" type rubber seals. They just plain wear out and get flat. The repair is a type of sealant that replaces the seals. And then there's the normal "lip type" seals around your crankshaft and moving parts. They too just tend to wear out. Again, even if you just had a rear main seal leaking and it was dripping on the ground, the labor to remove either the transmission or the engine is very labor intensive.
Most shops when attacking oil leaks, (including ours) will not just repair one leak. You as a customer may understand that we are fixing one leak. Then, 6 months later when your engine is leaking from another place, you question the previous repair. Oil leak repair needs to thorough and the customer needs to fully understand what ALL is actually being done.
Your smoke is most likely coming from your valve cover gaskets. The valve covers are above the exhaust
manifolds and when they leak oil, they tend to seep down the sides of the cylinder heads and onto the exhaust manifolds.
I have given you a brief synopsis of Cadillac's Northstar V-8 engine. You will get numerous opinions where-ever you go, because they are no-ones favorite engine to work on. It doesn't surprise me bit when you were told to just drive the car and add oil. It is just too much work for what it pays to repair. And, what does a quart of oil cost.
Even if you went to a shop and ask them to just replace the valve cover gaskets, you will find it to be rather expensive. Gaining access to the valve covers is quite a process.
Finally, in wrapping this all up, you need to weigh what your car is worth vs make a repair. Look at the whole car mechanically. Brakes, suspension
, tires, ... the whole works. And don't forget the trans. If you have a low mileage car, and it is in EXCELLENT shape, then you might want to considered removing the engine and having it resealed by a QUALIFIED tech. Don't cut corners and try and save money at some budget place. Your car and engine are too high tech for a person who doesn't know what they are doing. You will have issues you never dreamed of if you let someone work on your car who doesn't know what they are doing.
If your car is high mileage and pretty well worn out, you might want to cut your losses and move along. I have seen way too many people throw good money after bad. Even if you would put a new engine in, it wouldn't be worth anymore than it is now.
Good luck to ya.........