I've never seen a lower unit failure due to oil. From my experience, here's what kills lower units/gears:
1. Water intrusion - especially in areas with freezing temps in the winter.
2. BIG BUMPS - hitting something hard (reef, rock, submarine, etc.)
3. Fishing line/net - destroys the seals; see #1 above
4. Leaks - oil leaks out, even if water does not leak back in (see #1 above), gears go dry and fail.
5. Misadjusted or incorrect shifting settings that prevent full engagement in forward (or reverse) - causes bearing, cone, or gear failure.
So use whatever you like or trust - change it out 1-2 times a year - but check what comes out carefully for water or contamination! If you see ANY water, find the leak and fix it.
Look at the fill and bleed holes before you remove them for leakage - if you see any, replace the seal or o-ring on the plug.
Hope this helps - see ya on the water!