Why does my car not go higher than 50mph.

About 4 days ago my car started acting
up. Every time I drive the car far it does
this thing where I press the gas to go
faster on a 70mph high way but instead
of going fast it slows down. So I try to
press the gas more to see if theirs any
difference but it just doesnt go farther
than 40mph. Or if im on a 40mph road
and I press the gas it doesnt go farther
than 20mph. Ive been putting motor oil
every time it does that and seems to
make it better but I cant spend $15 on
that every single day... Btw my friend
went over a very high curb and messed
up my oil panel so its been leaking oil for
1 1/2 weeks dont know if that has
anything to do with it.
Please help..