Are you sure you're looking at the right cap? The cap on the engine is for engine oil, not transmission fluid.
In most cases adding a seal conditioner is not going to hurt anything but it's not going to help. If there is nothing wrong now, there's no need for additives that aren't already in the correct transmission fluid. If there is an existing problem related to worn or broken parts, no "mechanic-in-a-can" is going to fix that. If the lip seals in the clutch packs have hardened with age, a conditioner might soften them enough to get a little more life out of them but that's about it.