20w40 v. 20w50: does it matter?

Maintenance/Repairs
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27

OK, I see. I thought it was an 11 year old truck.

Here is your 15W50: https://www.walmart.com/ip/Mobil-1-15W-50-Advanced-Full-Synthetic-Motor-Oil-5-qts/20713647?wmlspartner=wlpa&selectedSellerId=0&wl13=101&adid=22222222227025388257&wl0=&wl1=g&wl2=c&wl3=77517661128&wl4=aud-273067695102:pla-107413214208&wl5=9023613&wl6=&wl7=&wl8=&wl9=pla&wl10=8175035&wl11=local&wl12=20713647&wl13=101&veh=sem

Then you could get a 10W40 as well if you just want a conventional oil.

These older engines are more forgiving to being a tad out of spec on the oil. I am not an expert on these but know they are popular. I would be worried about going too thin but maybe a synthetic Rotella T6 5W40 would be great in them. Remember that oils have come a LONG WAY since this truck was made.

Get on a forum dedicated to this truck/engine and ask them what the current oil is that they prefer for it.
Check out https://www.yotatech.com/forums/f116/rotella-t6-222240/ It seems the Rotella T6 5W40 is liked there from what I can tell. REsearch will tell you if this is OK or not.