My last oil change was less than 1,500 miles ago. I've been using Chevron Delo 15-40 LE (CJ). This time got Delo 15-40 SDE (CK) at Speedco in my Cummins ISL 400 powering my motorhome. With the SDE (CK) I find that once the oil is up to operating temperature my oil pressure is six to eight pounds lower than before at ~1,500 RPM. I consistently saw 48 - 50 psi before and now I see 42 - 44 psi. I can't understand it - 15-40 should mean it has the same viscosity under the same conditions, right?