1998 Buick Regal Cracked intake manifold

I got my oil changed and the employee told me that I had a cracked intake that was leaking oil. I had it repaired, but evidently all they did was replace the intake penlum they did not repair the lower intake because when I got my oil changed again the employee said my lower intake was cracked and leaking oil. I called Auto-zone and they said there is no way that the guy changing my oil could see if there is a crack in the intake manifold. Is this true? Also, is it difficult to change the upper and lower intakes myself if I have a manual?