[Pete]

Grease is not a good long term lube for leaf springs, even if the spring has a cover or is wrapped.
When grease gets old it dries out and loses most of it's lube properties.
It is almost impossible to apply effectively to an in service spring. It also attracts airborne dirt which is very abrasive.

Many people use Slip Plate for spring lube. It is a spray type dry lube that the base lube ingredient is graphite.
While graphite is good, there is something that is far better that coats to the molecular level.
It is Molykote. It combines molybdenemdisulfide AND graphite. It comes in a spray can and can be applied by brush also by just spraying on a brush and applying to such as model A springs. It has excellent penetrating qualities so readily flows between spring leaves if applied heavily to the sides of a spring. If you do not care about overspray, you can just spray it on very heavily and let it soak in.

And, no, I do not have stock in Dupont. I just use the stuff on parts we manufacture because it is good.