Now that the weather is getting hot I was thinking about using 15W-40 Delo 400 for my next oil change. This engine supposedly is sludge prone (www.allpar.com) and I was going to use this oil only in the summer months to help prevent sludge. Good idea? BTW the manual calls for 5W-30. I can't think of any possible harm a 15W-40 can do when the weather is hot. Thanks in advance for the help!