I took my 6 cyl. Mustang in for its second 5K mile service, and the dealer mistakenly put in 5W20 oil. I drove for a few days on this oil, then noticed the error on my work order. I returned to the dealer, and they redid the oil/filter change with the correct 5W30 oil.
My question is, could driving on the 20W oil have done any -- and I mean any -- damage to my engine?
Also, my owner's manual lists 5W30 as the correct oil type for my 4L V6, but it also lists 5W20 for the 4.6L V8. This probably accounts for the dealer mistake, but it also strikes me as strange. Doesn't a higher-performance engine generally require thicker oil?
The tolerances are so tight in new engines (last 15 years or so) that there really is no need for thicker oil, less resistance with thinner oil so faster spinning higher mpg.
GT uses thinner oil since it is more high tech newer design that the old modernized V6 engine.
On my oil cars 70-80's always used thicker oil in the summer, used to run 20-50W and 10-40 in the winter.
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2008 V6 Deluxe Convertible, Torch Red/Parchment top and interior, 5-speed manual, Exterior Sport Appearance Package
Mods:
K&N 63 Aircharger Cold Air Intake, JBA Evol Axle-Back Exhaust, Hurst Competition/Plus Short Throw Shifter,
Ford Racing Performance Parts front strut brace,
Ford Racing Performance Parts rear lowering springs,
GT Vert rear anti-sway bar
5w20 is usually jus synthetic or synthetic blend, it wont cause any issues, its recommended to use the 5w30 for the first couple oil changes for a new motor and then switch if you choose, high mileage motors usually stray away from synthetic because if there is an oil leak anywhere synthetic will find it
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2006 V6 Pony Package Premium -Mods: 61mm Powerhouse Turbo and Intercooler, Evolution Dyno Tune(Jon Lund), Steeda Performance Shorty Headers. Current Numbers: 350 rwhp/420 torque @ 8.3 psi (low boost)
415 rwhp/470 torque @12 psi Car died 6/23/09
New Set Up: 4.6 3v With Manual Trans, Bolt Ons
A friend of mine has a '98 Ford F250 with 260K miles on it. Compression in the engine is still good per his mechanic, but he said he's seeing a bit (just a bit) of seepage from the head gasket. Apparently not enough to warrant a head job.
Would switching to a thicker oil prolong the inevitable, and if so, what's the thickest he can get away with? The engine is a 4.6L V8.
A friend of mine has a '98 Ford F250 with 260K miles on it. Compression in the engine is still good per his mechanic, but he said he's seeing a bit (just a bit) of seepage from the head gasket. Apparently not enough to warrant a head job.
Would switching to a thicker oil prolong the inevitable, and if so, what's the thickest he can get away with? The engine is a 4.6L V8.
Thanks again!
its most likely not the head gasket. i would guess valve cover, oil pans are common too on the ford trucks. if it is a gasket, its gonna leak no matter what oil he uses. and speaking of oil, i just had my pro-high performance 5w30 syn-blend delivered today. ive been in a dead zone to get this stuff, but now the: Amalie Oil™- Wolf's Head Oil - XCEL Lubricants - Motor Oil - Transmission Fluid - Power Steering Fluid
Engine Oil Specs for '07 4L 
