The most likely cause for gasoline in the oil is a defective carburetor float and/or needle valve. When defective they fail to completely stop the fuel flowing into the carburetor when the engine is stopped and sets for a week or so. The fuel slowly flows from the carburetor into the engines combustion chamber and eventually ends up in the crankcase with the oil.
DO NOT run the engine with gas in the oil or it will be destroyed. Purchase a rebuild kit for the carburetor or bring it to a small engine shop and repair the problem. Change the oil, run for 10 minutes and then change it again to ensure there is no residual gas in the oil.
Kevin
For a Briggs and Stratton motor 3.5 motor used straight gas and oil in the crank-case, there is no ratio.
No you do not put oil in the gas - 4 hp Briggs & Stratton motors are 4 stroke, not 2 stroke in which you do have to add oil to the gas.
48oz
A Briggs and Stratton 17.5 horsepower engine can use any type of small engine motor oil. A heavy duty motor oil is recommended.
SAE30
Sae 30 oil
Briggs and Stratton recommends SEA30 oil.
right next to oil filter
There is no "gas oil ratio", the 3.5 hp Briggs and Stratton engine is a four-stroke design, oil should NOT be put into the fuel!!!
Ordinary motor oil the brand hardly matters.
SAE 10w30 full synthetic.
Briggs and Stratton oil must be used for Briggs and Stratton engines. They have both synthetic and standard oil. They have oils formulated for 4-cycle and 2- cycle engines.