oil

Is all lower unit oil basically the same ?  For my 25 Mercury. 

It was $11 at Fleet Farm for a quart of Merc brand. Stuff is getting up there. Even the SHipmate was $11 . Not that long ago it half the price.

Any outboard manufacturer's lower unit oil will work well. 

Agree with everyone's comments, such an easy thing to do, especially given the cost of lower unit repairs. 

Lifespan of your gear lube is not a concern for most people unless you are a heavy user, especially in a 25.  You should be changing your gear lube every year so unless you are a heavy user the lifespan of conventional vs synthetic isn’t going to mean much.

I dont think synthetic will do any good. Synthetic shines cause it can take a lot of heat before breaking down. I dont see the oil getting scorching hot from being in the water all the time . I have forgetten to  change the lower unit oil some years, but it is so cheap it is foolish to not do it every year. My 25 takes about 8 oz. or so. 

What kind if lifespan do you get out of synthetic  ? I've only used regular before. 

Most if not all oils Mercury supplies under the Mercury brand will also be available under the Quicksilver brand.

Under $10/qt at Walmart for conventional. They have it in synthetic also, don't recall that price.

Yes, there is Quicksilver, or Mercury lower unit oil. Some might even be synthetic. 

I use Quicksilver in the fuel . Is there Quicksilver lower unit oil also ?  I am looking for LU oil .

Like was said you can get some off brands at fleet farm but I notice its thin and different color so I just use the better quality Merc stuff. 

It’s only a 25, as long as you change it every year anything is fine.  However, as has been said, those small engines take such a small amount of oil that it really isn’t much more just to use the the good stuff and have a little better protection.  Quicksilver should be your cheapest OEM receommendation. 

The quicksilver bottle says on the front of the bottle one type is for 75 hp and below and the other type is for 75 hp and higher. I've only used quicksilver or mercury brands for the lower unit lube and never had a problem.

One shot, I have a 1997 25 horse Merc.  I have used everything there is as long as it is lower unit oil that is 80/90?. I was told to use Merc by the Merc dealer , (of course)  so I mainly use that. I have also used the cheap Fleet Farm ( Shipmate) and the dealer said it was poor quality.  SOme years I didnt even change it ( foolish) but  now I use Merc and sometimes Amsoil.  If I can keep it from overflowing , it is only about 8 oz. of oil anyway. Very cheap  maintenence. 

I believe that one uses 80w-90 gear lube. I recommend getting the Quicksilver brand at WalMart. Oil is cheap compared to new lower unit. 

Almost 30 years old. Lost manual somewhere in several moves .