Valvetrain complete, but a few ???'s

I Have The Same Deal With My 318 In My Duster With 80,000 Miles. Good Oil Pressure Any Other Time, But For The First Two Seconds On Start Up In The Morning, It Has No Pressure. I Can Hear The Valvetrain Clatter, But Immediately After That, It Gains Full Pressure And Is Fine. I Run A K&n Filter, Which Is Supposed To Be Good, But Maybe Not. It Seems To Be A Drainback Issue. I Run Valvoline Vr1 20w50, As I Live In Cali And It Never Gets Cold.