squishy lifters.

im woking on a 70s dart with a 318. it sat in some ones backyard for many a year and is now running again.

it seems as though some of the lifters wont pump up and are squishy. i dont wont to dig any further than the valve covers. so im wondering if i can add something like marvel mystery or some atf of something that will clean up and help fill the lifters for proper operation.

any one got ideas?
is atf in an engine bad?