User caterham used to blend m1 euto 0w40 and m1 afe 0w20 for a nice blend.
I tried it and worked good in oil burner k24 honda... ran smoother for sure.
Now that m1 afe 0w30 and m1 esp 0w30 are out kinda pointless to me.
But choose what you like ... doubt your motor will care.
I'd go with 5w30 or 10w30 to start and maybe 0w40 or 5w40 if its an oil burner.
Had a 2000 dakota with that 2.5 ... gotta drive it hard to move but not horrible.
That gearing will help .. i ran wider but lower profile tires for the same effect.. noght and day difference.
You could go with plantinums or copper if needed.
My accord is a 1qt/1k burner ... for last 200k miles
I put in oem iridium's and they last 50k or more.
Looks good so far ... probably still has a lot gunk in heads and/or timing chain area.
I'd keep it up with 5k oci.
Coworker is restoring that body truck as restorod ... rare bird these days.
Why not get a few 5 qt jugs of normal 5w30?
Wally has qs synthetic for $15 each now or chevron 5w30 for $14...
Or 6qt boxes of havoline 5w30?
I'm sure that Buick would run forever on any of those.
Havoline himi semi synthetic 5w20 and/or valvoline maxlife 5w20 would do great.
Mines at 280k ... run 5w30 now ... 5w20 is noisier but car doesnt care.
Havoline is good oil ... ive used planty of himi semi synthetic 5w20 in my accord.
Chevron supreme is also good.
Try it and see what you think ... as long as it meets the specs your motor wont care.