Black or Gray Oil Coming Out of Upper Exhaust Vent &/or Lower Unit Holes :  Many motors are basically used as a secondary motor, and usually then as a trolling motor, which are ran for considerable length of time at a lower RPM.  The fuel oil mixture ratio of 50-1 is probably too rich for this type of usage. The black oil that seeps out of the motor is just unburned oil from the gas mixture.  The black color is coming from the carbon created during the pistons firing situation and is accumulated unburned inside the exhaust housing.  When the motor sets for a while the excess unburned oil in the fuel is mixing with and softening up the carbon, creating the black oozing oil.  You can reduce the amount somewhat by properly adjusting the carburetor idle jet to it's leanest position, also where the motor runs best at an idle. 

   This black or gray oozing oil could be also because of a stuck open thermostat which would not allow the motor to warm up.  However thermostats that get stuck usually do it in the closed position, so a bad thermostat is usually not the cause of this condition.

   Or, it could be a 6 gallon tank that the fuel line is still attached to the motor during storage while the outside ambient temperature is high enough to expand the fuel inside the tank to where the gas is forced into the carburetor and IF the needle valve is leaking, it will force fuel into the engine and out the exhaust which mixes with the exhaust carbon, then leaches out as BLACK oil out the prop's exhaust.  

   If you are getting a thick creamy gray oil oozing out the prop hub, it is essentially the same as above except the inside of the housing may have less carbon build up, therefore the color will be lighter.   It is the same thing as described above, but possibly from a new or rebuilt motor that may not have been run much for some time and there is not a lot of carbon inside the exhaust housing.   If there is minimal carbon built up from previous runnings, the oil will be the thick creamy light gray color that has a lot of water mixed in it.   On the 9.9/15 hp, this could also be caused by the converging ring (#332395) is missing on the prop.  This plastic ring facilitates a suction from the outside of the prop when the motor is running at a slow speed to help suck more of the exhaust gasses away from the "thru the prop exhaust" when idling in neutral or running slow.

Here black oil is running out of the upper exhaust vent of a 1992  9.9hp after the motor was used for trolling using a 75-1 ratio semi-synthetic & then sat for 3 months	Here a creamy colored oil is exiting the motor's exhaust housing.  This is basically the same concept as the black oil except probably in a cleaner motor, BUT notice the converging ring missing on this prop allowing a oil build up inside the housing

   This oil seepage may not be evident until a day or even a week after usage, then will seep out up to a couple of months later.  Most repair shops just recommend that for most boaters to then simply live with the idea that you may have to wipe it down after each usage. 

    You could also see this oil seeping out of the front lower unit water drain hole.

    It is observed that if these motors are used on a smaller boat as the only propulsion, and act as a "get there" motor and also as a trolling motor, this black residue is considerably less to almost non-existent because the motor gets hotter because of the faster RPM, where the motor has a chance to warm up, thereby "burning" some of this excess oil.

   Also if the engine was winterized with fogging oil then you would expect to see oil coming out of the exhaust because this would soften any carbony buildup in the exhaust system.   If it smells like gas  =  unburnt 2 stroke oil.   However if it smells of rotten eggs  = Lower unit oil.

    This dripping oil many times may be confused with the lower unit oil leaking because of the color and location of the oil coming out.   Another reason as to this also happening is as explained above under the Converging Ring on the prop, is if this ring is missing it can hurt by not creating enough suction to pull the oily residue out of the exhaust housing at low speeds.

    Akin to the above situation, where spark plugs have continually become fouled, I have been experimenting with using 2 gas tanks for the last eight years (2009) now and for me, I am happy with the idea.  One, a 3 gal. tank, mixed at 100-1 with full synthetic oil and it is used  ONLY FOR TROLLING, the other is the 12 gal. main 50-1 tank also used for my 70hp outboard jet on the same boat.  Again, I am using the 100-1 synthetic only for trolling with the 9.9.  If and when I need to run with this 9.9 motor faster, I simply switch to the larger 50-1 tank.  However the winter of 2007/2008 in an emergency flooding situation, where I was rescuing a lady and her son who were stranded in her house with the water was still coming up fast, my main 70 hp jet water intake had become plugged with floating grass or hay, making it useless.  It was getting dark and we were caught is a strong very swift current pushing me broadside to a fence, I did not have time to change the fuel tanks as I was broadside to the current, with the 9.9 then my only source of power then.  I ran this motor at full throttle during the last half of this rescue mission for probably 20 minutes before I got to dry ground and offloaded the passengers.  As of 2013, I am still using this motor with no apparent harm.  But I was using the FULL SYNTHETIC oil mix.

   I got the above idea from the newer VRO oil injection motors.  This VRO abbreviation stands for Variable Ratio Oil injection.   The larger motors using it (not the 9.9 or 15hp however) electronically sense the different RPMs then automatically adjusted the oil injected ratio.   They put out a 50-1 ratio at high speed and a 100-1 at slow speeds.  Recently the motor manufacturers have gotten away from this VRO system and went to a direct constant ratio pump.  This apparently is because with the VRO on a large motor with multiple carburetors, if you are idling, or trolling at the lower ratio fuel mix, so you decide to pick up and run, the carburetors still has the bowl full of that low ratio mix, the engine will then be UNDER lubricated until fresh richer fuel is drawn into the carburetor.

   I have kept track of the run time at trolling with this 100-1 ratio with one particular motor over a four year period and can account for 43+ hours of only trolling, with only one a sputter, all it took to clear that up was to speed the motor up until the spark plug became un-fouled.  Then 4 hours later, back home I changed the plugs before the next run.   In 2009, I switched over to using a 75-1 ratio of a semi-synthetic oil mix with the idea of not having to worry about changing tanks, but I get the black oily residue as seen in the photo above, so possibly back to the 100-1 full synthetic ratio for all my running.

   You will see responses on some of the major outboard motor forums being very adamant to not deviate from the factory 50-1 ratio.  I realize that many of these knowledgeable persons, (many who may be old line mechanics) who may have seen blown engines in the past which may have been attributed to poor oiling.  However oils have changed dramatically even in the past 10 years.  Others on these forums expressing their concern may be just repeating what someone else has said.  And you have to consider that anyone who posts on a public forum does not want to stick their neck out, make a recommendation only to be chastised if the results turn out negative for whatever reason.  That is the way of life these days where liability's head pokes itself out.  That said, I have done the tests ON MY MOTORS for a number of years and you have read the results above.  I am happy with what I now use, but if your situation is different, then make your own decision.

    I many times use the boat in bays which are somewhat salt water, initially I used to switch tanks to the 50-1, when I use the muffs to flush out the trolling motor at the end of the day.  The thought here is to use the richer gas to leave a better oily film inside the motor when it is stored until next time.  I now think that the full synthetic oil is so much better, that I don't bother changing, however when flushing, under these conditions, I do NOT KILL the motor with the kill button, but choke it to kill it.  This will tend to also increase the fuel/oil inside the motor if I do let it set for a while because where I live, as I normally do not fog my engines for winter storage.