Drying oil makes sludge and blocks the proper function.

Oil is designed to not only lubricate hot metal on metal as it moves in the engine, but also capture and suspend the tiny fragments of metal fillings and scraped glaze that would otherwise pile up on every level surface in the engine. This concoction of items along with burned oil creates a "sludge" which is hard for the oil pump to handle. Regular oil changes get rid of the old oil with the suspended particles before it gets too thick. There are products that can flush the oil system, but you have to be careful to follow the instructions and not leave these additives in too long or you will damage your engine. If sludge is a problem you need to change your oil and filter more often then the normal recommend time. Check your owner’s manual and use the worst case scenario.

You get flushing oil in the UK but is seldom used or required with modern engine oils if changed often enough

Contamination comes from the Rings allowing "Blow By" under the the force of the "Explosion" in each cylinder. The Rings are constantly Expanding against the walls of the Cylinder in order to create Compression of the "Fuel / Air" mixture for an Explosion to happen. This "Metal on Metal" gradually wears the parts out & when the Explosion occurs, some of the Hot Gasses & Particles get past the rings & down into the oil in Ur Crankcase. Natural Oils Will Burn & Oxidize, getting thick with Varnish causing the Oil to form the thick "Sludge" U hear about. We used to only have "Non-Detergent" Oils which were VERY BAD at doing this. Todays "Detergent" Oils are not as prone to do this, but still will if driven too Long. This oil carries these Abrasive Particles to the moving parts causing even More Wear the longer they are driven!!! In Todays Technology, there is NO REASON to use "Natural Oils" when "Synthetic Oils" are So Available & Superior in performance!!! Everybody always remembers the "Weight" (10w.-30w.) for their car but have never heard of the "API" Standard or know what it means! Too BAD because it is More Important than the "Weight"!!! "A"utomotive "P"etroleum "I"nstitute "Grades" the "Oils" by what is in them, ie. ADDITIVES!!! Such as "Anti-Foaming, Sludge, Friction Modifiers etc.". Every bottle or can of Oil has an "API" ratings "Circle" on it somewhere. The ratings are for 2 types of Engines. Diesel (Compression Ignited) and Gasoline (Spark Plug Ignited). They start the "Rating Scale" @ "A" ( the Poorest Additive Package) & go Up from there. So if U drive a "Gasoline Engine Car" that required a 10-30w. Oil it would be labeled 10-30w. "API" "SF" for Example in Ur Owners Manual. So when U go to buy Oil U need at Least that "Rating" on the Package! If U drive a Diesel powered car it would be 10-30w. "CF" for "Compression Ignition Engines". Synthetic Oils are "Built" not "Cracked" (broken down) like "Natural Oils". And, they Virtually Never "Breakdown" & turn to "Sludge!!! Neat huh? In Fact, they actually Clean "Sludge" out of Ur Engine!!! And as far as Protecting it is Superior in reducing Wear, Transferring Heat, Ease of Starting etc.. I bought a New 1988 Land Cruiser & after 3000 mi. & put Synthetic in it everywhere! Engine, Transmission, both Differentials, Drive Shaft & Wheel Brgs.. After 500 mi. I had gained 6 mi. per gallon!!! I changed the "Oil Filter" every 5K mi. & added just enough to show "Full" on Dipstick (1qt. or less)until I had 1000,000 mi. on it then I threw it out & put all new in. When I sold this Cruiser it had 300,000 + miles on it & Emission Tested Cleaner than the day I bought it!!! The Only Repairs I had to make were light bulbs, belts & plugs!!! I sold it to a mechanic friend who is now approaching 500,000 mi.!!! John