Briggs calls for and from my own personal experience the RC12YC in the Vanguard will misfire. Requires the RC14YC which is a hotter plug. I use the NGK BKR5E here at the shop
If you have the heavy duty cyclonic filter which is part number 820263 you are probably fine. If you have the smaller round filter it should be part number 692519. There is a similar filter part number 694018 that looks identical but will cause rich running within about 2-3 hours of installation.
Both issues can lead to improper running, lack of power, and the misfire can cause gas to wash down the cylinder walls and dilute the oil which can cause the oil to be thinner, and also when the gas in the oil in the oil in the gas gets hot, it vaporizes and is pushed through the breather back into the intake, and will act rich running similar to if the choke was closed.