Yes you can, as a mater of fact that is what is done in two stroke engines that dont have a sump.
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.
To achieve a 30:1 fuel-to-oil ratio, you need to determine how much oil to add to 500 ml of gas. For a 30:1 ratio, you would divide the amount of gas by 30. Therefore, you would add approximately 16.67 ml of oil to 500 ml of gas.
If you mix the oil and gas together in the proper mix and do not add oil to the oil resivoir you will be fine.
An Oil Injected motor will have a separate Oil Resevoir, so mixing Oil and Gas is not required, and in fact inadvisable.
The amount of oil you should add to 4.5 liters of gas depends on the specific oil-to-gas ratio recommended for your engine. Common ratios for two-stroke engines are 50:1 or 40:1. For a 50:1 ratio, you would add 90 mL of oil, while for a 40:1 ratio, you would add 112.5 mL of oil. Always refer to your engine's manual for the correct ratio.
Look on the Gas cap of your weedeater. It should tell you the gas to oil ratio. If its not there try looking in the manual or online.
32:1 is 32 parts gas to 1 part oil... for every gallon of gas you add 4oz of oil... usually refers to 2 stroke engines.
16oz
Look on the oil container and the engine that you want to use it in. The engine will tell you what ratio to mix the oil and gas at. The container of oil will tell you what amount of gas should be mixed with it. I have found that the best way is to put the oil into an approved gas can and then add the gas to it. This helps to mix the oil better.
To determine how much oil to add to 20 gallons of gas at a 50:1 ratio, you would divide the number of gallons of gas by the ratio. For 20 gallons, you would calculate 20 gallons ÷ 50 = 0.4 gallons of oil. Since there are 128 ounces in a gallon, this is equivalent to 0.4 x 128 = 51.2 ounces of oil. Therefore, you should add approximately 51.2 ounces of oil to 20 gallons of gas.
To calculate the amount of oil needed for a 50:1 ratio, you can divide the total volume of gas by 50. For 20 liters of gas, you would add 20 liters ÷ 50 = 0.4 liters of oil. Therefore, you should add 0.4 liters of oil to 20 liters of gas at a 50:1 ratio.
2.5 Ounces it say the mix rate on all 2 stoke oil containers. Add the oil then the gas to get a good mix.