i have a nissan pickup,that smoke at start up.after setting overnight.i;am guessing. it valve stem seals.do they make anything in a bottle,that will help it.
No. Nothing in a bottle will help. The only cure is new valve stem seals.
Try one of the high mileage oils such as Valvoline Max-Life on the next oil change. These have additives that slighty recondition the old/worn engine seals to reduce oil burning. If this has no effect, then the only fix is to replace the seals.
Check the oil label under the hood if there is one there or the owner’s manual. You may be using oil that is too thin. Thick oil won’t stop the smoking at start up if your engine is really old, but it might help one that has some life in it.
Just being curious here for a moment.
What year truck? How many miles? What color smoke? Does it start right up, run fine, and no stumbling?