Xender,
The Saito 72 is set super rich at the factory in an apparent effort to keep you tinkering with the needle valves long enough to allow the new parts to get adequately lubricated at a speed of less than 4000 rpm for the first 10 minutes (see instructions). It won't even run at high throttle with the glow plug disconnected. Adjust the idle needle first then adjust the high speed needle just enough so it runs without the battery connected to the glow plug. From then on its a juggling act of adjusting both needles in leaner increments during break-in.

Black oil is bad. Stay away from black oil. You can watch your engine's life bleed out in black oil.

On the subject of fuel leakage, you may find that fuel still leaks out even after you replace your tank as correctly suggested by others. It may continue to leak out of the engine due to the high placement of the tank relative to the carburetor. Since the UCD 46 cowl covers my Saito 72, I use a Sullivan filler valve to fill the tank. This valve requires no special fitting to fuel it, just a short length of brass tubing. Between flights I just stick a plugged piece of brass tubing in the valve to block it.

If you want to you can just let it leak out since it just dribbles out the back of the cowl and kills some kinds of grasses and makes an ugly mess on paved pit areas. Maybe other club members won't say anything to you about it.