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Impact screw driver...best invention since duct tape! You can shop around and get one for $10 or less, I think Harbor Freight has it for 7 or 8 bucks. If you need it right away Craftsman makes one, will be a few dollars more. It is a heavy driver that you hit with a hammer while you are putting turning torque on it.
__________________ Stratos 2500 wa
Twin 225 Johnsons
What is that sucking sound?
An "impact driver" will do it easily. It is a 'screwdriver' like device which you put in the slot, turn it in the 'loosen' direction as far as it will go (to take up the slop) and then hit with a hammer(moderately). Each hit will cause a few degrees of rotation of the screwdriver bit at the same time it is being pushed firmly into the slot. This will get out the most stuck of screws.
They aren't expensive and with the right adapters can work on slotted, phillips and with sockets as well. Sears sells them as well as others.
Regardless of what you use, make sure it fits in the screw slot absolutely tight. You can do that by grinding or filing the end of the screw driver to expose a larger section of the blade.
I had the same problem with mine, on an evinrude. I got a screwdriver that was the right size for the slot, and then I used a few different types of penetrating oil, and tapping the butt end of the screwdriver with a hammer. All it takes is a little patience. tap it and walk away, do that a few times and it will come right out.
BTW, when you do get the bolts out, don't lose the nylon washers that are on them. Sometimes they come off on the bolt, sometimes they stick on the hole,
__________________ Stratos 2500 wa
Twin 225 Johnsons
What is that sucking sound?
Whatever you do take your time and don't wallow out the head. Also buy new nylon washers when you go to replace it and also, Overtons carries replacement bolts that take an allan wrench to avoid this in the future.
I have to give mine a little tap with an impact screwdriver every time on my Yamahas, and I normally have to change the Lower Unit oil about 3 times a year based on hours.
You can use a lug wrench, the "X" type that has three different size sockets and one flathead screw driver, if you don't have an impact driver. File off the flathead screwdrive to fit the slot. You can twist it with both hands while pressing it into the bolt with great force. I do like the idea of a bolt that can be removed with an allen wrench. It would be nice if all the lower unit bolts were the same size.
When re-installing the new drain plug, use a Craftsman flathead bit. It's hollow ground & fits the slot width perfectly.It's 3/8 drive so you just pop it on the end of a ratchet or mini breaker bar.I have used this on many motors and never walked the bit out tearing up the head.
Right tool for the job.
hope this saves you future problems
I couldn't get mine off a 2002 Mercury 225, wouldn't budge. I went out and bought a screwdriver with a square shaft instead of a round one. Placed it in the slot, put a wrench on the shaft, loosened right up when I turned the wrench.
Lower Unit Oil Bolts Stuck 
