I prefer a stick since it will dry much faster. Using a lotion or cream allows your dog to lick it right off which defeats the purpose. Make sure its water proof.
A small amount will not harm your dog. I've checked with my vet and others and they all agree. My boy has a section on his nose which is a light pink. Anytime he goes into the sun it turns a bright red. I know its got to be painful. The sunblock has really helped.
J