I need to change the oil out of the lower unit. What kind of oil do I squeeze in and about how much is it? Thanks in Advance

serial # to the mercury 500 50 hp is 2738854

You can buy the oil (qt) for about $20 at Wal-Mart and a "pump" kit for about $8 bucks. I used the quick silver high performance. Alternatives are cheaper. There was a great thread on here a week or so ago on how to change it. I do mine and it takes 10 minutes on 150 hp. Easy...

You will use a gear oil and it's pretty complicated to do if you haven't had any experience I would recommend looking at the owner manual if you don't have that look online for previous post or blogs no luck there order manual or take it to the boat doctor it shouldn't cost to much and they can probally get you in and outta there good luck

Diddo what shake and bake said. Walmart and Academy sell the lower unit fluid and the pump kit.

And changing the fluid is easy too. Just remove the top screw, then the bottom screw, and allow the oil to drain out. Make sure that the oil coming out is not a milky color and there are no metal shards in the fluid that came out. If no metal and no milky substance, you're good to refill your lower unit.

To refill: connect the pump to the bottom drain hole and fill the lower unit until oil is coming out of the top hole. Replace the top screw and then quickly remove the pump and replace the lower screw. Done!

+1

Here is a video on step by step instructions if you have the time and place to watch it. It is very good

http://www.youtube.com/watch?v=gkU-xdNW-3M

how often is lower unit oil suppose to be replaced in general?

Edit: watched the video..nevermind

every year in the fall. and u are supposed to use the gear oil that is specified for your engine

Yea, what he said....don't go into outboard maintenance just willy nilly! If you've ever been stranded on a lake with bad weather coming up, it can be VERY dangerous.

aint nothing in there but gears , and roller bearings..
any gear lube will do , 70-80-90 weight ..lower weight= easier cold shifting
there are some better brands that will mix well with water .

If you check it for water every once in a while, by draining a little into a pan , them add a little to make up for what you lose ..a quart will last a long time..

problem is , fishing line , or something else ruins the seals and before it is checked, it runs on just water ...then oops!

It doesnt matter if it is fall or winter/spring/summer
check it often, dont forget to pull the prop and check for line around seal.biggest cause of failure

another lower unit failure is caused by a bent propshaft ..
allows the seal to wobble lets lube out /water in ..
water is not your friend in the lower unit

Dont forget to put new orings in every time...

This is true. I know first hand about the bent prop shaft letting water in through the seal. Lucky for me I caught it before the freeze

I think Mercury High Performance G/L Is the best. You can run half gear lube and half water and lower units would not fail under a load during performance test. Not all gear lube is the same. Thats like saying Pennzoil and Valvoline are the same! Wrong I use valvoline. Don'nt be that guy putting walmart oil in your $40000 boat. Change your gear lube before the winter!!!!! Change your l/u seals everytime and use the blue ones. Part # 19183 The yellow seals I think are not as good as the blue ones.

If you are talking about the drain/vent plug seals..
it is a good idea to use new ones..

however in my mercury , I use the nylon evenrude washers.
they can be re-used several times..
mercury has one time only seals, or used to , I havent bought any in years.

If you are talking about the shaft seals, prop and shifter shaft
I stand corrected (I only change these when there is a problem)

Mercury has come out with a yellow seal that you are suposed to be able to reuse , but I dont like them at all. I would use the evenrude washers before the yellow merc seals. My thoughts are: the lower unit drain plug gasket seal is about a $1.oo each ! For piece of mind replace each time. You can feel the squish when tightened properly and it reminds me of a crush ring on a new spark plug. It only squishes right once.