The relief valve and the bypass valve are two different things. The relief valve is set around 78 PSI. This is in the oil filter bracket, before the filter. It dumps back into the oil pan. The bypass valve is internal to the filter and IS actually 13-18 PSID (PSI Differential). (I was just going off the top of my head in my first post, sorry). The bypass valve is, just as it says, a valve the bypasses the filtering medium when the pressure differential across it is too high (13-18 PSID is the spec, see
here ) So even though there is a filter after the potentially contaminated cooler, the filter will at times be bypassing oil (meaning the engine is getting unfiltered oil, but dirty oil is better than no oil in most cases)