Do Accords like Thick or Thin oil?

I am going to be doing a 6 day, 7500 km run accross Canada next week. I have a 99 Accord 4 Cyl with 100k kms on the clock. Right now I use Mobil 5w30
with slight consumption (ie. 1 Liter/7500 kms). I understand that Mobil is a "thin" 30 weight and have been considering making the switch to German Castrol 0w30 which is a "thick" 30. Has anybody used this oil before or even a 40 weight in their 2.3's? Will I experience any real drop in MPG and will I cause any harm to my engine by using a slightly thicker oil in my car? I am trying to strike a balance between Valvetrain protection and MPG. Any input is greatly appreciated.
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if you have valvoline there where you live i recommend that over brands. it lubricates well and it is better than castrol. i used to use castrol in my old civic and the engine would have a slight knock until it warmed up and when i used valvoline once because of circumstances the knocking wasn't there. it's excellent motor oil.
jeff

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