This is the first place oil arrives under pressure--crankshaft main bearing journals (except the 351C where cam bearings get oil first). Pressure must always be maintained during engine operation to keep a liquid wedge between journals and bearings. Rule of thumb is 10 psi of oil pressure for every 1,000 rpm. Oil pressure is lost when bearing/journal clearances become excessive. It also can become lost with excessive pump rotor clearances or defective pressure relief valve operation.