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  1. #1
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    New 300XS owner with questions

    I was referred to this forum based on my search for information on my new to me 2007 Mercury 300XS Optimax that is on the back of my Shearwater Bay Boat. From my initial research it seems that these motors are pretty reliable, the motor has just over 200 hours. What issues should I be aware of with this motor. I was told to run either Rec 90 or Super 93. Where can I get a service manual for this motor, as Mercury as it listed as NLA? Where is the best place to get parts on the internet? I've noticed that this motor has many different options for the lower unit, I do I tell which one I have? I'm pretty handy, so I do like to perform my own maintenance (lower unit oil change, impellor, filters, etc.) and leave the tech items that are above me to the experts at my local Mercury dealer. Any guidance is greatly appreciated.

  2. #2
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    Mercury Service Manual part number is: 90-8M8020345 I think that they are still available. They are pretty reliable once they get thru their initial shake down. Some have had air pump and coil issues. Run 91 R+M/2 non-ethanol gas. Mercury has a new 2 stroke oil #8M0078011 that is recommended for the 300SX. Also a new gear lube #8M0078016 for Sport Master gear cases or any HP use. There are Sport Master and Torque Master gear cases available on 20" and 25" mid sections. Sport Masters are unpainted with low water pick-ups. Torque Masters are painted black. There is also a service manual for the gear cases #90-842009. Working on them is really straight forward and the manuals are really good. I have been on a steep learning curve with them. I don't know it all, but if you have a question, PM me and I'll try to help. David

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  4. #3
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    Quote Originally Posted by Davidlake View Post
    Mercury Service Manual part number is: 90-8M8020345 I think that they are still available. They are pretty reliable once they get thru their initial shake down. Some have had air pump and coil issues. Run 91 R+M/2 non-ethanol gas. Mercury has a new 2 stroke oil #8M0078011 that is recommended for the 300SX. Also a new gear lube #8M0078016 for Sport Master gear cases or any HP use. There are Sport Master and Torque Master gear cases available on 20" and 25" mid sections. Sport Masters are unpainted with low water pick-ups. Torque Masters are painted black. There is also a service manual for the gear cases #90-842009. Working on them is really straight forward and the manuals are really good. I have been on a steep learning curve with them. I don't know it all, but if you have a question, PM me and I'll try to help. David
    Thank you, I will definitely PM you with any questions. I have the Sport Master gear case

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    those numbers work, found the manual on Amazon

  6. #5
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    Merc says some ethanol is OK for this, i forgot the percentage.Most important thing: NEVER let the gas go bad from storage. They are sensitive to getting plugged injectors, both air and fuel. Run Starbright Startron or Stabil Marine, or whatever you like. And make sure you last run already has the preservative in it, so anyresidual fuel in the motor will have the additive in it.

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  8. #6
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    Quote Originally Posted by swaddict View Post
    leave the tech items that are above me to the experts at my local Mercury dealer. Any guidance is greatly appreciated.
    The most important piece of guidance you can get is not to assume that the guys at your local Mercury dealers are tech experts.
    Markus' Performance Boating Links:
    www.toastedmarshmallow.com/performance

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    Quote Originally Posted by swaddict View Post
    Thank you, I will definitely PM you with any questions. I have the Sport Master gear case
    I am very surprised they put a SM on a bay boat. Was it rigged this way or was it repowered?
    Dave
    **********************

  10. #8
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    Quote Originally Posted by dogeatdog View Post
    I am very surprised they put a SM on a bay boat. Was it rigged this way or was it repowered?
    it was rigged this way

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    I just saw on Merc website that up to 10% ethanol is OK for 300xs. Still very confusing though on which oil they recommend as best for it.

  12. #10
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    Quote Originally Posted by Sleek Jr View Post
    I just saw on Merc website that up to 10% ethanol is OK for 300xs. Still very confusing though on which oil they recommend as best for it.
    See post #2 for Mercury Racing two stroke oil. That is what Mother Mercury recommends.

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    Oh I guess that's their NEW recommendation? I just looked in both my factory Merc manuals for a 2007 300xs, and they recommended Premium Plus in both, and said "If Quicksilver oil is not available then be sure to use a TC-W3 rated oil".

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    Yes, There was a Mercury Tech Bulletin for it.

  15. #13
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    I'm finding that the new "Mercury Racing" oil 8M0078011 is kinda hard to find and way more expensive than Premium Plus. Also just read over on BBC forums that this new oil should never be mixed in the oil tank with ANY of the older types.

    Also there is some debate that if you're not using your 300XS like a "race" motor (extended high rpm WOT), but spend a lot of time low RPM, fishing, trolling etc, you might be better off running the DFI or Premium Plus oil.

    If anyone finds a good source to get the new oil at a reasonable price, please post here.

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