Hello Mr Briggs,
I think on the Northstar I would stick with the 10/30. That engine is designed to get oil to stay in the cylinders and 10/40 wouldn't solve any of the problems you are having. If the problem is in the sending unit, a thinner viscosity would only make it worse, and if the problem is with deposits we need to go the route of getting the system cleaned up.
Quaker State is a good oil, the only brand I see alot of problems with is Pennzoil-it has something to do with paraffin I believe but I know it makes a mess especially if maintnenance isn't kept up. I recommend synthetic whenever possible, RedLIne is my preference. Ammsoil is a good product but I have had better experience with RedLine products.
I need to retract what I said in my earlier post about Slick50-it is actually a PTFE or Teflon additive and there has been much debate over its benefits as well as problems with it clumping up in some engines. Molybdenum treatment such as Restore are friction modifiers and reduce the friction between parts-resulting in reduced wear and increased efficiency. What is really interesting are the nano-technology treatments that are coming out now. I haven't had enough experience yet to recommend a certain product, but I know of several and I have seen some amazing tests. If you are interested I can forward you some info I have on nano treatments. Also if you are interested I can forward you more technical info on the oiling system features in the Northstar engine, the engineering allows it to run at 6000 RPM's for extended periods but also lends itself to common pressure issues such as the one you are having and also there are very common problems with oil consumption.
Let me know what happens after you get the sending unit changed out, and I would definitely recommend doing an oil system flush, like I mentioned, you may be able to get it done as a service at a shop, or there are canned products that work very well also.
You can keep posting to this question until we get it resolved, and any future questions you need help with you can request me and I will be notified. Keep me posted, I'll be watching for your posts.
Thanks for using JA