The exact purpose of transmission additives varies from product to product. Still, the overall concept is that mixing them into your regular transmission fluid during a top-up somehow increases the efficiency of your car’s functions. For example, some additives claim to soften and condition old, hard pressure seals, while others claim they can completely stop fluid leaks within the gearbox.
Do these additives actually do any of this stuff? They do, at least for a time. When first added to your car, additives can cause a small but noticeable improvement in your car’s function, reducing gear slippage, unclogging filters, helping to realign gears, and so on. The problem is that these additives generally don’t last very long. They often go stale well before you’d need to top up your regular fluid, breaking down amongst the intense heat produced by your gearbox’s components, forcing you to open it up and add them again if you want to keep the effects. This means that additives can actually increase the burden of topping off your fluid instead of reducing it.
