Kinda Random Transmission Question

[RinkyDinkyZenki]

So I bought a 5 speed trans recently and I'm planning to swap my car soon. But in the meantime I wanted to keep the gears and everything inside nice and lubed up to prevent any rusting because the trans was left out in the rain once I believe. But anyways what I'm asking is I put ATF in it to keep the internals covered. Will that do any harm to it? Whenever it's time for the swap I'm gonna drain it all out and fill it up with some cheaper gear oil, run it for about 100 miles, then drain and fill with some higher quality gear oil.