My dad wants to use Delo 5W-40 turbo diesel synthetic oil in our 2006 Mercury Grand Marquis. The oil is rated for gasoline and diesel vehicles, but the owner's manual says to only use 5W-20. My dad says that it should be fine but I am concerned that it will be too thick and possibly cause other issues.

Any ideas

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Current: 1975 450SEL, 83 300D, 88 Yugo GVX, 90 300D OM603 swap, 91 F150 4.6 4v swap, 93 190E Sportline LE 3.0L M104 swap, 93 190E Sportline LE Megasquirt, 03 Sprinter, 06 E500 4Matic wagon.