Hmmm, I'm also a skeptic about putting anything into the engine besides oil. I think that if i were to use anything to clean out the gum and Sludge is a highly reputable additive from a store. Ya ive also heard about using Kerosene, Transmission fluid, and diesel fuel. Seriously though, would you want anything that didn't have a lubricative properties in your ENGINE?
I think that ultimately the best way to clean out the valley is to pull the manifold. Yes thats a lot of work but the best way because you KNOW its "clean" when your done. If there is still sludge up there then that will hinder oil flow back to the pan where its needed! LOL.
Yes i understand that your not racing this engine and a few engines that ive seen were running "just fine" until they were torn down had crude oil stuck to everything. One i saw needed a scraping with a putty knife to start the cleaning process. UGH!!!
Id start by changing your oil and filter with a plain oil that you can afford to throw away fairly soon.( id really just use your normal oil, but in my case that would mean throwing $50 of Mobil 1 away in a 1000 miles) With that oil change try something like this. Rislone Engine Treatment
Then just check it around 500 miles and then see what you think? If its dirty then its doing the job. Change it (oil&filter) again, then do another 500 with another bottle of cleaner. Repeat till the oil stays clean and then go back to your "good" oil.
Ultimately if you don't get your oil pressure back ( assuming you have a good gage your checking with) then i think its time to pull the pan and install a melling High Volume oil pump. That should get your oil pressure back to normal.