I drove my Mazda 3 for 90 miles on low oil level (according to dipstick reading) without the engine ceasing?

If it was just below the 'add oil' mark, and things did not make funny noises or overheat , then you should be OK. Most cars have a 'check engine' light that will flash if you are dangerously low on oil. (or anything else that could be terminal) If it did not come on, that is good news.