will a additive to stop a oil leak work? will it cause any other problems?

it is on top back to the rear of manifold they told me. the mechanic said it was leaking a little bit on the exhaust . I have a few drips under the car. I drive it 4 times a week to work apox. 40 miles round trip al highway. its not using a lot of oil I check it each week on refill of fuel. the mechanic said they top off the coolant but if they did it was low so I topped it off. I have it in for radiator cooling fans that are not engaging and turning on. my temp gage went from normal setting up high dropped down to to right in the middle than back low again. like I said the mechanic says it has a small leak, he wants $ 750-800 to replace seal, the car has 138000 on it . I don't want to put a lot of $$ into it so that's why I ask if additive will jack it up more? it did sit for 18 months while I was on a deployment for the military. driven a few times not a lot though I guessing the seals could of dry up a little bit. well I hope this helps a little bit . thx for your help and reply