20w50 is what people use when they have a worn motor or one put together with two much clearance, They also use 20w50 for severe duty,race cars etc. You should not have to use 20w50 in order to have decent oil pressure. If thats the case you have a problem. I would not except a motor like that . After you try their suggestion, then its time for action if they dont make it right. Action being a letter to them with a copy to the Better Business Bureau. Once again ,give them the opportunity to fix it. If they dont its time to see the Judge.