Chevy truck 6.0 liter question

[71-LS6]

I think I've heard this discussed here before. Last fall I bought a used 03' 2500HD chevy with the 6.0 gas engine. I bought it as a back-up tow vehicle to my VERY UNRELIABLE 05' Dodge Cummins. The Dodge continued to give me fits, gas has been cheaper than diesel, and they get the same mileage, so I've put 25,000 miles on the little Chevy in 5 months. It's got 95K on it now. I love this truck, it pulls nearly as well as my diesel (when the diesel is not broke) and it runs like hell when it's empty. Here's the question;

I've changed the oil in it at least every 6,000 miles since I bought it, it doesn't use oil and it doesn't smoke. For the last 3-4 weeks, I hear a very distinct lifter rattling noise at start-up. Noise never lasts longer than 1-2 seconds. Did I cause this by using plain old Castrol 10W 30 for oil? Will changing to a lighter viscosity oil help at this point?