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.
Mixing Oil & Fuel for Small Engines. 1  4 Stroke Cycle Spark Ignited Engines: Do not mix oil in gasoline, or modify engine to run on alternate fuels. 2  2 Stroke Cycle Spark Ignited Engines: Always mix a high quality, 2-cycle oil, such as Briggs & Stratton lawn mower oil, at a 50:1 gas to oil ratio.