Using the information you have provided and not visually inspecting the roof, the black spots on your roof are more than likely algae. Most shingle manufacturers use copper or zinc coated granules to aid against the growth of algae.
Two-stroke oil (also referred to as two-cycle oil, 2-cycle oil, 2T oil, 2-stroke oil or petroil) is a special type of motor oil intended for use in crankcase compression two-stroke engines.omparing regular lubricating oil with two-stroke oil, the relevant difference is that two-stroke oil must have a much lower ash content. This is required to minimize deposits that tend to form if ash is present in the oil which is burned in the engine's combustion chamber.
1 If the engine has a single fill port for both the small engine oil and gas, you have a 2-cycle engine.  If the engine has two fill ports, one for gas and another separate fill for oil, you have a 4-cycle engine. Do NOT mix oil and gas in these engines.