How do you do a specific gravity test on the battery with oil in it?

First of all make sure the fluid level is full or to the top meaning that the oil is about 1/8 inch below the bottom of the vent tube well. If it is low just add water, preferably distilled water until the oil is about 1/8 inch below the vent tube well. Then you will have plenty of electrolyte below the oil to take your reading.  All you have to do is take your hydrometer and plunge it down as far as you can go with it, usually until the top of the plate,  then squeeze the ball so you get the air and maybe the few drops of oil that might be trapped in the tube from sticking it thru the oil. By releasing this air it will blow any oil that might be trapped in the tube back to the top of the battery.  Now release the ball and you will suck up the electrolyte below the oil layer and get your reading. It’s that simple. Watch the Hydrometer Test Video on the Hydrometer Test page link from home page.