3. What are the GLS products and why doesn't Pennzoil have equivalents?GLS stands for Gear Lubricant Special and this is usually a product available only from the original equipment manufacturers. General Motors has several GLS products, and various other manufacturers also have their own. In order to meet our customer's needs, we now offer GM Synchromesh fluid which can be used in place of GM P/N 12345349. At this time, this is the only GLS product equivalent that we offer.