This is from memory so 'your mileage may vary', multi-weight oil is 
     made of a base which is the lowest number plus additives that increase 
     the viscosity when hot. These additives are expensive and the larger 
     the variation the more required and the harder it is to maintain the 
     higher viscosity. These additives break down eventually and you 
     approach the base weight of the oil. i.e. 10 wt. for 10/40 oil.
     
     Jeff

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>   I guess I still have a ways to go before the big road trip in August . I'm 
> going to try a heavier wt. oil, right now I'm running the 10w40. I've checked 
> the relief valve , though when I took it out it seemed to of been in the

I was concerned about oil press. in my Jetta TD after installing a 
gauge in the cylinder head (where the low pressure sensor was). I 
found that 10w40 oil gave the lowest oil pressure when hot - about 5 
lbs less than 10w30, and 15lbs less than 20w50. I've used 20w50 ever 
since, and get 30-40 psi at the cylinder head when hot @3000 rpm. 
Idle pressure runs about 10-15 psi hot.