1993 Ford Festiva using fluids

I LOVE THIS LITTLE CAR but its using tranny fluid.I do not see a leak but when it shifts from 1st to second it slips..if I add fluid to full level its ok for a couple of weeks then its down about half a quart I tried some Lucas Tranny seal and that didnt work it started slipping from 2nd ..its always 2nd that slips. sometimes it dosnt slip for weeks then it strts if i keep fluid full its fine but uses it quick.... what kind of stop leak do you reccomend and what kind of tranny fluid should i be using i cant afford a rebuild..can a faulty tranny make the car run hot and give me no heat??