I have an older Evinrude and have been using the Mercury outboard gear oil for years without problems. On some other sites I see guys saying to using Mobil 1 synthetic, Royal purple synthetic, Lucas synthetic, Redline, Valvoline, etc. Should I switch to one of these synthetics or stay with Merc Lower unit oil? Thanks.

Stay with the horse that got you here.   No need to fix something that isnt broke.  If it runs good and you dont see or hear any problems then why change it?    Theres nothing in the world wrong with using conventional gear oil.  I still do too.  200 Johnson runs fine.  And my motor is 22 years old.  

 

That a my .02 but I'll leave it to the motor heads on here.  

Thanks Teal, that was my thought as well. I just wanted to be sure I was not missing out on something possibly better.

Brand doesn't really matter as long as it meets the correct specs. I use Walmart Supertech oil in my lower units. 

4 hours ago, rboat said:

I have an older Evinrude and have been using the Mercury outboard gear oil for years without problems. On some other sites I see guys saying to using Mobil 1 synthetic, Royal purple synthetic, Lucas synthetic, Redline, Valvoline, etc. Should I switch to one of these synthetics or stay with Merc Lower unit oil? Thanks.

They sound like engine oils. The lower unit needs an 80/90 wt. gear oil. I use the Walmart stuff. Check the specs and you will find it is right up there with the more expensive lube. 

11 minutes ago, Dogface said:

They sound like engine oils.

They do - but at least the Mobil 1 and Lucas also have gear oil (75+ wt) in their synthetic lines.

The rules for gear oil are different than engine oil.  The primary reasons synthetic oil is superior are 1) it flows better in freezing cold temps (which most of us don't deal with in boating) and 2) it has additives which help extend the life of oil seals and hold in soot/other solid bits better (which don't typically occur in a gear housing).  Unless you're seeing metal bits come out when you drain it, I would just keep using what you've been using.  And if you are seeing metal bits come out, I wouldn't expect a synthetic oil to solve that problem.. it would help suspend those bits better (and not allow them to cause as much damage), but you'd still have a problem to fix.