If you smelled rotten eggs then most likely you will smell them again and you cat is clogged/all used up.
Besides just being old there is one thing that will kill a cat converter, raw fuel. If your cat goes bad it is most likely because something else is wrong, an 02 sensor is bad, the fuel pressure regulator went out, your MAS is not working properly, etc...
Does your 4.0L leak oil? If yes the Lucas can slow these down
If you do a compression test on your engine before and after the addition of some Lucas you will see an improvement. However I dont really consider 150K mile high miles on a 4.0L, 200K is getting up there, 150K is just slightly used as far as the shortblock is concerned.
Your valvetrain may need some work, but as far as piston rings and what not you are fine. You can crack open a 200K mile 4.0L and you will still not see much wear inside the cylinder walls, they are bulletproof.
Here's the deal. If you want to add lucas go ahead. It is an excellent product and will not hurt anything, But you dont need to add much to your crankcase. When you use Lucas it's all about how much you use.
If you use 3 quarts of lucas and 2 quarts of oil you are going to stress your oil pump big time and do more harm then good.
If you use 4-1/2 quarts of oil and 1/2 quart of Lucas, now you are talking. The lucas will stick to the engine internals and help reduce wear upon startup. The lucas will also help lubricate any worn seals and bring back some lost compression.
read the back of the LUCAS bottle, its got you covered.
Oh and there isnt a Ford engine out there that doesnt benefit from synthetic oil, you wallet benefits the most because it more then pays for itself in gas mileage....