A look at my sig shows the reason I ask. I have a 15 F150 5.0 that turns 5 the first Week of September. Only have 38k miles that are high percentage highway. Contrast my DD 17 Subaru 3.6 with over 60k. My question concerns the F150. I did the front and rear diff along with a trans pan drop and transfer case drain/fill at 30k. Front/rear diff's have Mobil 1 75/90 in them. I would like to change them again at 100k. That would be around 8 years of that fluid in the locker. I don't think any issues with the diff's but wanted to ask your thoughts. Transmission will get a pan drop and fill every 30k that's easy peasy. I assume it's a good idea to do transfer case same time as trans. Plugs is a big Going to 100k miles is going to be a 10 year timeline. That's a long time for a plug to be in there. Might be an ordeal to get them out. I have nightmares over this as I had a 04 F150 with the 2piece plugs in the 5.4 3V. Yes I know it's not that disaster of a plug anymore. So, change em out early? And if so when? 50k? 75k? The truck runs great and no mechanical issues to date.