Frigidaire FEX831FS4 transmission leaked oil. Should I refill? With what?
Don't buy the correct oil if it's that expensive.

Use a high viscosity automotive gear oil typically used in differentials, and just buy the cheapest one you can find in stock at your local auto parts store. 

Buy SAE 140 or SAE 110. Those monograde oils should be the least expensive, but if they have a bad selection or a sale then a multigrade oil like 85W-140 would be fine too. Really anything they have there will work, even if it's a 75W-90. If you already have some gear oil in your garage from changing the differential fluid in your vehicle, then just use it.