Your transmission fluid is fluid for your transmission system and power steering fluid is fluid for your power steering system. To be more specific, your transmission fluid is responsible for cleaning up your transmission. At the same time, it enhances rather than reducing the friction between its moving parts.
This liquid is heat-resistant and "gluey" in nature, which means it adds friction between the clutch pack's different components to get better grips from them. This is in contrast to motor oil that reduces friction between moving parts instead.
Meanwhile, power steering fluid, like motor oil, is a lubricant that reduces friction between various power steering system parts.
To Mix or Not To Mix, That Is The Question!
- For The Most Part Don't Mix and Match: It's a bad idea to substitute a lubricant with a friction enhancer. Don't do that because in the case of transmission parts, they'll lose their grip while in the case of your power steering, its parts will instead jam. However, because both fluids are hydraulic fluids. Therefore, the power steering fluid should work for a little while on your transmission system. However, in the long run it will affect the gearbox and the seals of the pump in the long run.
- Why Is It Sometimes Okay to Mix and Match? It's better to use approved fluid for your specific car system since it's better to be safe than to be sorry. You should use transmission fluid for your power steering pump and you shouldn't use power steering fluid on your transmission pump. However, there's an outlier to this rule. You can use your transmission fluid in the power steering pump of vehicles made back in the 1970s or earlier. They were less sophisticated systems back then so you can get away with it.
- Modern Cars and The Wrong Fluids Don't Mix: The reason why modern cars made after the 1970s and later are sensitive to fluid mixing is because they use lightweight and advanced materials, specifically if we're talking about cars made in the 1990s and after the Turn of the Millennium. Transmission fluid on 1970s power steering systems will treat the substance as just another hydraulic fluid type and it won't have issues with its gluey, grip-boosting nature since it's working with fewer, cruder car parts.
- Sophisticated Fluids Service Specific Parts: To add transmission fluid to your 1970s power steering is like using a stapler as a hammer. You can technically do that, but it's more efficient to use a hammer and the stapler isn't fulfilling its intended role. Modern cars have more delicate parts and subtle engineering differences. Each of their systems requires fluid and oil specifically designed for these parts. Using the wrong fluid is asking for trouble, like using a stapler as a hammer for softwood or particle board. You'll break the wood before you can hammer down the nail!