Full synth is good for life if you change the filter on a regular basis along with regular top ups (same brand, same type, don't mix and match)

I've seen underground engines that have run the same synth oil for their life-span, just change the filter every 1000 hours. Just did an overhaul a few months on a old cooper 8 cyl diesel that was built in the 40's and has been running the same synth since it's last overhaul in 94' thats 13 years on the same oil, almost 100,000 hours on the clock!

08' Jeep Wrangler
64' Plymouth Sport Fury 383
58' Triumph TR-3A