importance of overflow valve
Presently manufactured oil filters have top by-pass valves (under the cover) or bottom by-pass valves (built in the bottom of the filter cartridge). In filters with bottom by-pass valves, the oil must flow round the filtering cartridge and the can bottom at the time when the valve operates. The sudden oil flow may cause washing out gathered contaminants which may lead to excessed working engine elements wear. The effect may be even worse due to the fact that the valve opens more frequently when a lot of contaminants accumulate. When the engine starts, the by-pass valve in filter opens and contaminants are pushed into the engine. Top by-pass valve placed in the filter excludes effect of washing contaminants out since the oil flow does not flow round the filtering cartridge during the valve opening. Therefore the engine is better protected from contaminants which can't be taken out of the filter once it gets there.