I am going to tell you what to do, weather or not you do it is up to you, others will disagree, but I have personal experiance with a similar problem with a different car that I had.
Switch to HD30, yes thats right, HD30. Straight 30 weight motor oil. It is thicker than 10w40 and will stay thick even when hot, it will not be to thick to pump through the engine. Trust me on this, lower end knocks can be cured for a long time with a thicker motor oil, I have done it.
Back in '95 I bought a brand new Oldsmobile and I used HD30 from the very first oil change to the last one that I did, even in the winter time. 150k later the car ran as good as new, used no oil and was as clean as a whistle inside the valve covers.
I had a '77 Pontiac with the 400 big blockin it and I bent a rod by overheating the engine and driving it with the temp gauge pegged at 250F for 5 miles. HD30 and I drove that car for two more years and 30k before it finally gave in.
You have nothing to loose, if you want to stay with full synthetic than use Castrol Syntec 20w50. But trust me, HD30 will do the trick.

 

Thanks for the recommendation, Spike.

So how is HD 30 different from 5W30? That's what I put in it when I changed oil two weekends ago.

 

Well to put it in simple terms, 5w30 is a "5" weight motor oil with enough additives to make it protect like a "30" weight motor oil. They do this so that it reduces friction inside the engine, the EPA says that a engine must be as efficent as possible all the time, less friction inside the engine means better MPG. We are talking only fractions better, but better none theless.
Youve seen oil commercials, "better protection against thermal breakdown". Thermal breakdown is when the additives in the oil break down, the oil never goes bad, it is the additives that go bad, and it becomes contaminated with deposits and combustion gases and stuff.
But in theory, if you had a good enough filtering system and could add the additives backin the oil you would never have to change it.
A Heavy Duty 30 weight motor oil will not break down like a multi-weight motor oil, it will get dirty and need to be changed every 3,000 miles just like any other dino oil, but it is a true 30 weight oil, not a "5" weight oil trying to be a "30" weight oil.
The first number on any oil bottle is the actual weight, or thickness, of the oil, the "W" means it is rated for winter use and the second number the type of oil it is trying to mimic.
What happens to oil when it gets hot? It thins out. So what happens to a "5" weight oil when it gets hot? If you have a engine that burns oil, a thicker oil will slow if not stop that. Low oil PSI? A thicker oil will help that too.
Sludge forms when the oil breaks down and isint changed in time, the additives that are broken down stick to the inside of the engine and start to build up. If a engine is prone to sludge it is most likely because the wrong oil is being used. You need the right oil for the application. Without going into to much detail it is the EPA's fault that Rover's are prone to sludge, that and using the wrong gas. Using the wrong gas will change the internal engine temps, to hot and the oil will break down to soon.
Anyway, did I answer your question? HD30 is thicker than 5w30 and will not thin out as much.
If you want more info than this shoot me a e-mail.

 

Also Dave, that is why you cannot use a multi-weight oil in your air cooled lawn mower engine, it will break down to fast and shorten the life of your mower.