Mixing oil weights

Lifter noise on cold start is caused by one of two things; a lifter that does not work correctly (most likely), or a lack of oil flow to the lifter. If it's the lifter, nothing that comes in a bottle will fix it. If it's a lack of flow, a higher cold viscosity index number will actually make it worse. The viscosity number is an oils resistance to flow, the weight is 7 pounds per gallon regardless of viscosity.