I have had a couple negative expierences with 30W oil.
After coming home from Vietnam, 1970, I bought a 1966 Plymouth from my Aunt, 40,000 miles on it.
The car had nothing but 30W Quaker State oil, at 3000 miles changes.
At 42,000 miles, I was changing the oil, and new valve cover gaskets.
Under the valve covers was so much almost dry sludge, I could barely see
the valve springs. I dropped the oil pan, and the same situation was there. The screen on the oil pump was in very bad shape. No amount of 5w would have broke loose any of this sludge.
I bought a S10 truck, 4cyl, 79,000 miles, from an older guy, that the engine had seized up. I removed the oil plug, but no oil came out. After removing the engine, I pulled the oil pan, and over 7 qts of oil came out. The screen on the oil pump was completly cover, starving the engine of oil. Talked to the older guy, he had used nothing but 30W oil from the time the truck was new..
My theory is that thin oil, 5W, will not remove any sludge, and 30W will only add to a bad situation. Everyone to their own opinion.....