thicker oil flows slower than a thin oil, but the oil pump moves a constant volume. What happens is the pump always moves a constant volume of oil for each revolution. Becasue the thicker oil flows slower but the pump moves a constant ammount of oil for each revolution the thicker oil has to be pumped at a high pressure to move the same volume of thick oil as it does if it were a thin oil.
Just incase the oil pressure gets too high, there is a relief vavle built into most engine oil systems (usually inside the pump) to keep the pressure from getting dangerously high.
There is also a relief vavle inside most oil filters to pretect the filter canister from rupturing due to high oil pressure. What this vavle does is allow oil to bypass the filter element and go directly to the engine. The idea here is that if the pressure in the filter gets that high then there must be a clogged filter so dirty oil is better than no oil.
