I was going to change out the spin on filter for the first time yesterday, and a lot of fluid came out when i started to spin it off, so i tightened it back up, not wanting to loose all of my fluid. How much should drain out when I do just the spin on?

I didn't have enough ATF to refill the whole transmission, plus its super expensive. Thanks.

Go ahead and take it off you wont lose but maybe 1/4 quart its not going to drain whole tranny. dont forget the magnent that is on top of filter it might stay up on filter nipple you will need to pull out and clean and reinstall.

1 pint/.5 quart

http://www.allisontransmission.com/servlet/DownloadOnDemand?ApplicationID=155&DownloadID=5
Fluid Loss With Filter Change Only:
When performing initial main filter change or changing main and lube filters at recommended intervals, approximate fluid
loss for each filter as follows:
1000 and 2000 Product Families Transmissions
Control Main Filter = 1 pint (0.47 liters)

Thanks, it was even less than that, pretty easy to do.