That story about the intake manifold doesn't sound right to me. When does the oil go anywhere near the intake manifold?
I'd look at a bad head gasket or bad piston rings as the problem. Either one might show up when you do your compression test. I'm leaning towards a head gasket, since your car is running so poorly. You didn't say what type of engine you have. The running poorly definitely has something to do with the poor fuel economy. I'm not so sure that the "fuel in oil" problem is related, but it may well be.
I'm not sure how you can tell that there's gas in your oil. Have you had an oil analysis performed? Blackstone Labs in Ft Wayne, IN, does them for about $25.
http://www.blackstone-labs.com/
I don't usually pay any attention to those oil pressure gauges- it seems that they're wrong more often then they're right.