The only risk with using ATF instead of oil is if you have water ingress, the ATF will 'foam' a little more than oil. ATF is a modified hydraulic oil. One of those mods is more detergent, to help keep things clean. If it caused problems to seals, why would it be used (for years and years) in automatic transmissions?
Merc specified ATF in the trim pump for many years. It's only when they repackaged 30W oil into a bottle labeled "Power steering and trim fluid" did they change the spec from ATF to 'Merc power steering and trim fluid'. It's a marketing execise to get people buying 'their' oil, and not just any tranny fluid you can pick up in walmart (at 1/4 of the price).... Your ATF will do you well for many years to come.
Chris.......