Automotive lubricants and additives are two different things, typically. So, any product which is to be added to a lubricant to enhance it's properties should be considered an additive and placed under this category.
Ranked Sites

Sells automotive additives and chemical products, including Militec-1 metal surface lubricant, Duro-Care for leather, vinyl and rubber and GTA fuel enhancer. Product info and online ordering.

Offers IXL metal conditioner products for fuel and hydraulic and lubricating oils. Product and distributor information available.

Independent representative selling automotive fuel and lubricant additive products. Mail order.

Offers a metal treatment for engines and transmissions designed to improve performance and protection. Product information available.

Offering a very complete line of gas and diesel engine treatments with extensive testing information and testimonials available.

Canadian based seller of a metal treatment for engines, and transmissions. Also offers lubricants pre-mixed with their metal treatment. Product information and online ordering available.

Offers a PTFE engine metal protectant designed to improve engine protection and performance. Product information and online ordering available.

Metal engine and drivetrain treatment products for gas and diesel engines designed to improve protection and performance.

Car care lubricants and appearance products for automotive, industrial, and consumer applications.
4
1,402,591
NA
18,600,000
731
441

Offers a motor oil and automatic transmission fluid additive designed to improve performance and increase protection. Product info and online ordering.
3
1,166,082
NA
105,000
851
463
1