You're in the driveway, hood popped, transmission fluid ready to pour, do you add it with the engine running, or kill the motor first and hope for the best? It's a common question, and getting it wrong can mean overfilling, underfilling, or a slippery mess. So here's the short answer. For automatic transmission vehicles, you should leave the engine running when checking or adding transmission fluid, switch it off for manual transmission ones, but there's more to it than that.
The more common automatic transmission, works under hydraulic pressure, and that pressure only builds when the engine is on and the oil pump is running. If you check or fill the fluid with the engine off, the reading on the dipstick won't be accurate since there won't be any hydraulic pressure for the oil. You could think you're low and overfill it. The right move is to park on level ground, start the engine, shift through all the gears to circulate the fluid, then put it in Park (P) and check the level. Only then should you add fluid slowly, in small amounts, rechecking between pours. Even sealed transmission units need a fluid change, although at longer intervals.
Not every car plays by the same rules. Some transmissions don't even have dipsticks anymore, especially on newer vehicles. In those cases, checking and adding fluid usually requires a professional or for you to locate the fill, and inspection plug, and definitely more than a funnel and guesswork. Even if your car does have a dipstick, some manufacturers want you to check fluid hot, others cold. Some say idle in (P), others say Neutral (N). It's annoying, but it matters.
That's why your owner's manual or service manual should always be your first stop before you even pop the hood. It'll spell out whether the transmission needs to be warm, running, level, or all of the above. Adding fluid the wrong way can cause issues and overfilling the system can aerate the fluid causing foaming, reducing its ability to lubricate, and lead to excessive pressure. Getting lazy here can lead to hard shifts, whining transmission, and reduced fuel efficiency. And have some heart, even good cars with bad transmissions deserve a loving fluid change, it won't make a crappy gearbox any better but at least it'll keep things rolling.
Transmission fluid isn't like engine oil, it doesn't burn off under normal conditions. They can last a long time and most transmission fluids are recommended to be changed between 60,000 and 120,000 miles. If you're consistently low, you probably have a leak. Look for brown or red spots under the car near the transmission pan. If it's dark, smells burnt, and is accompanied by a noisy neutral, slipping or hard shifts, you've got bigger problems than just topping off. Transmission fluid is usually a bright red that deepens a bit with use, overtime. Dark brown or black indicates contamination and a light pink, on the other hand, means coolant contamination. And yes, some transmissions have coolers and that's where the coolant can come from.
When adding fluid, use the exact type specified by the manufacturer, so consult the user manual to double-check. Transmission fluids are not interchangeable, and manual transmission fluids are thicker than automatic transmission fluids, as they serve different purposes. Using the wrong one can lead to grinding noises from the transmission, difficulty in shifting and slipping gears, basically messing with shifting performance. And as mentioned before, never overfill. Engine running or not, get it right because when it comes to transmissions, close enough is typically not good enough.