I read somewhere that the oval shaped dishes are not really bigger so to speak, only that the oval design gives each LNB the 18" of surface needed to see multiple satellites at the same time so each LNB "sees" enough signal. I cannot guarantee the correctness of this statement, though.
Here are some answers to your question: " I have the dtv oval slimline hd dish with lnb. can i use it with a FtA reciever? "
- if it's on the roof already, and you just plug into it, no
- if you want to buy one 'cause it's cheap, no
- it won't work "out of the box"
- if you have one for free, and you don't mind modifying it, and you aren't expecting the best performance, then: maybe