I purchased a Mazda 3 last August. The manual specified a break-in procedure for the first 350 miles to drive under 65 MPH, keep RPM below 4000, do not use cruise control and try to do a mix of city and hwy driving. After the first 3000 miles (about late December) I changed the oil & filter. As expected there was a lot of shimmering "glitter" in the oil. A magnet swirled in the drain oil collected into a gummy film in short order. I installed a magnetic drain plug at the first OC.
As for transmissions and trans-axles which are not exposed to products of combustion, I usually do a flush & fill every three years. I've thought about installing magnetic drain plugs in the transmissions and will probably do that this year. Since I have full access to a machine shop, embedding a magnet in the end of drain plug would be a trivial task.
Ray