Thicker oils don't guarantee better performance but increased engine resistance; here's why.
They don't flow easily throughout the engine to lubricate compact spaces and cause the engine to pump them with more pressure.
Because the engine pumps them with more power, it consumes more fuel, reducing the mileage and increasing the frequency of your fill-ups.
Thicker oils become frosty under glacial weather conditions, which may cause your already old engine to fail to an even worse extent.
If your engine is compatible with 5w20 and you pour a 5w30 in it, you are causing it to work and wear out more, leading to more damage.
Thicker oils may buy you a bit of time with your high mileage engine, but that time won't end in its favor unless you're pouring compatible—no matter thinner engine oils.