Is your lawn mower using more oil than it should? Are you not seeing any leaks or no smoke coming out of the exhaust?
Any internal combustion engine is going to use oil over time. As your engine is running, oil will seep past your piston rings and be burned up in the combustion process. It can also evaporate as well. It really doesn’t matter whether you are using conventional or synthetic oil as they both will burn or evaporate over time.
If the oil usage is excessive, then something must be done to correct the problem. The lawn mower engine must be given a leak down test to determine where the oil leak is coming from. It could be something as simple as a valve guide oil seal or more complex like the piston rings. A leak down test will tell you what the problem is and get you on the right track to troubleshooting this problem.
Once you know exactly what the nature of the high oil consumption is, then you can repair or replace those components and test the engine to be sure that you have resolved them.