Transmission fluid is primarily composed of a base oil, which can be a type of lubricating oil, and various additives that enhance its performance. While the exact composition can vary between different formulations and manufacturers, roughly 80 percent of automatic transmission fluid (ATF) is typically made up of base oil (which is largely lubricating oil). The remaining components are made up of additives, such as detergents, dispersants, and friction modifiers.
So, in answer to your question, 80 percent of transmission fluid can generally be considered as lubricating oil.