I maintain all my vehicles. I just recently started using full synthetic engine oil though. I have all high mileage vehicles and always did conventional oil changes at 3K. No engine issues on any of them. I definitely keep an eye on the levels often. Also being high mileage I went from the spec of 5W20 to 5W30 on them. I have the following fleet in my household, 03 Ford Focus with 290K, 04 Ford Expedition with 180K, 03 Ford Ranger with 240K, 10 Hyundai Sonata with 210K and a 17 Mazda 3 with 98K (always put 0W20 full synthetic in that with 5K interval). I have aluminum crush washers and use them on every change. You can definitely see the indentation left on the old one. Good luck lining that back up if not replacing it. Same for the trans pan.

I do trans filter and lubricant usually around 30K intervals. The Hyundai has no access to pan and filter so it's just drain and refill on that. The others get pan drop with new filter and gasket. The Mazda is due for a change, interesting that you say keep the trans filter in for every other fluid change. I would prefer to do that. Do you use the Mazda blue fluid? It has BG Products full synthetic in there now that is Mazda spec approved but is the red fluid. In the Fords I have always used Mercon V. The Hyundai I use the Valvoline Max Life. I'm trying to consolidate keeping so many different lubes. I have researched where people use the Valvoline Max Life in place of Mercon V and also in the Mazda for the FZ spec as the oil meets them. Even some Mazda dealers don't use the factory oil and use the BG Products. Valvoline Max Life has the Mercon V and Mazda FZ spec.