^^^There are 5W-20s which cost more than most all 5W-30s out there.
(And YES, I know that you were referring to him wanting to 'use up' the 5W-20 he already owns vs.
buying new 5W-30 of any grade.)
But yeah, unless you are leaving your car out all night in International Falls, in the dead of winter (and maybe even then depending on the particular 5W-30, i.e.; whether or not it is on the thicker, or thinner end of the allowed 5W-30 viscosity range), you will be fully OK.