350 crate motor problems

I have a '50 Chevy 3/4 ton p.u. with a 350 ci crate motor installed. It is the stock 185 hp version, nothing fancy. I took it on it's first long trip (400 miles) after it had 3,000 miles on it. It was mainly freeway cruising between 55 and 80 mph for most of the day.
On that trip it went through 4 quarts of oil, got 10 mpg and at the end was running really bad. I pulled the plugs and found that the rear plugs on each side were fouled so bad you couldn't see the electrode. The other plugs were not fouled but looked like they had a brownish buildup on the insulator. I am looking for ideas as to what could be wrong here.

First I'd check warrantees. Next do compression test on each cylinder. If they are all the same then the rings should be fine, if not there might be broken rings or they are not installed correctly. If the compression checks out it might be valve guides. Guides will allow oil to flow into cylinders and burn and foul plugs. Hope this helps.

First I'd check warrantees. Next do compression test on each cylinder. If they are all the same then the rings should be fine, if not there might be broken rings or they are not installed correctly. If the compression checks out it might be valve guides. Guides will allow oil to flow into cylinders and burn and foul plugs. Hope this helps.


I checked on the warranty, it has a 3 year, 36,00 mile warranty IF (a big IF) it is installed in a vehicle that was originally equiped with that engine. I already did the compression check, everything checked o.k.
On to the tear down. :disappoin Thanks.