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.

i bought a suburban that was suposed to run on 5w30.it came out of arizona and evidently the guy thought he needed better oil pressure so he ran 10w40.it ate the oil pump.the ls engines use an oil pump like a transmission pump.it mounts on the front of the crankshaft.and they do not like thick oil.so i would shy away from it.

I suggest you ask the question on crownvic.net. If you're not a member, join. It's like this forum only for the Ford Panther platform. I'm sure among all those Panther owners, someone has tried it.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.

I've owned and driven a large number of Crown Vic's, mostly cop cars.
The general consensus is that 5w20 is the best oil for those motors. Oil that's too thick causes the timing chain and guides to wear prematurely.

I don't think using 5w40 would cause it to explode immediately, but Ford designed the motor for 5w20 and that's what I always used- even on ex police vehicles with over 225,000 miles, the Motorcraft 5w20 worked great. I can't remember who makes the oil, perhaps Kendall?

Crownvic.net is a weird place.

'better oil pressure' - is the bump on the 5 to 10 end or the 30 - 40 end that would cause the better pressure? If it is on the low viscosity end, then 10W30 would do it and be closer to spec, alternatively 5W40 wold be closer to spec that 10W40. I am guessing 'increased pressure' would come at the upper end, the spec relative to higher temps.

I always think its best to follow the manufacturer spec specially for viscosity and specially the last set of numbers on an oil container.

the heavy 40 hot viscosity oil is twice as thick as the 20 one, This might show as high pressure but low volume being transferred (bypass piston closes earlier - saps more hp - saps more mpg too) - we need a high volume of oil to get to the engine parts to cool them down sufficiently. btw whats the problem with regular dino 5w20 oil which is available nearly everywhere. Its meant for the engine and its proven to keep it in good shape.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)

I knew a guy that wore out his Mustang motor in 72,000 miles. The mechanic told him he was using the wrong grade of oil.

According to the mechanic (and I don't know much about Fords so I am just repeating what I have been told) the oil return holes in the heads are too small for the higher weight oil to run though at a speed needed to supply the oil pump. Therefore oil tends to get trapped on top of the heads and is not getting back to the oil pan fast enough to be pumped in the correct volume.

He started using the correct grade of oil and after nine years he still has the same used engine that was installed to replace his original.

Theres some controversy around that theory. 5w20 oil is popular with a lot of car manufacturers for vehicles sold in the US. Outside of this market they specify higher viscosity oils for the exact same engines in the same cars. They specify 5w20 here because the EPA is pushing them to improve fuel economy via the lubrication system. They're trading off long term wear for a minuscule bump in fuel economy. In a climate like Atlanta, theres no reason why 5w40 would hurt this engine.

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CENSORED due to not family friendly words

It's just that I have like 33 gallons of the stuff sitting in the basement. HE has decided to just go ahead and use 5W-20.

Which leads me to my next question: I have 3 MB diesels that don't leak or burn any oil that I could put it in:
my 190D with almost 320,000 miles now
my brother's 300SD with 181,000 miles
and our Sprinter with almost 185,000 miles now

We have ran conventional in my 190D the past 2 oil changes (10w-40 then 5w-40, 3,000 mile interval instead of the recommended 5,000 miles) but according to the PO, it was ran on synthetic Rotella.

The 300SD seems to have been ran on conventional by the PO and we have not changed the oil on it yet (haven't driven it much).

The Sprinter has been ran on synthetic Mobil 1 Turbo Diesel Truck oil (5W-40, changed at 7,500 miles instead of 15,000 miles as recommended by manual) since new. My understanding is that the Mobil 1 oil is a higher grade synthetic, but should that really even matter if we change that oil at half the specified interval?

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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.

It should be fine. Oil analysis would prove it.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.

Some real world Mobil 1 stuff.....

About four months ago I was asked to help tear down a 1970 280SE with the fuel injected 2.8. The new owner had just bought it from the original owner and it had been sitting with a start-up now and then for about twenty years. Something to do with the original owner dying and now his wife died and so on.....

I asked the guy to start it up and he insisted on tearing it down for a rebuild first. I pointed out that was not the normal order of things and he asked me to look through a fistful of records just to see what I thought of the car from that standpoint. I noticed that the engine had been rebuilt at a cost of $7,800 in 1980 and that every oil change after that was with Mobil 1.

The car looked like it had been sitting outdoors for many years, and the engine area looked like a former crash-pad for homeless rodents. But after removing a bit of linkage and hose we pulled the valve cover.

Everything inside looked new. I mean, shiny from the factory new. So we replaced the valve cover and after a manual turn of the engine gave it a boost and it started.

And after a bit of rough running it smoothed out. I guess the old gas had to purge from the injection pump.

So does Mobil 1 protect an engine from wear? Well, we didn't measure the cam wear (because we could not see any) but it sure works wonders in keeping the inside of your engine clean!

I've heard of that controversy and have done my research. The older modular motors called for 5w30. This was before 5w20 was commonplace. The simple answer is that the motor was designed for thin, 20 weight oil, and thicker oil actually accelerates engine wear in that motor. I've talked to Ford engineers, city fleet managers, and have owned probably 15 Ford modular v8s. I've heard personally of two oil related failures: chain guides worn out after running 15w40 for 50,000 miles and 75k miles - separate cars.

What exactly does "wore out" mean? How did the mechanic determine what was wrong? Was a teardown done?

I just pulled off the valve covers of my GM 350 L98 motor. The holes are pretty large (at least 3/8 inch if I am viewing it right)

Doesn't say much. Without a teardown, how would anyone know why it failed and how the oil helped?

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01 Ford Excursion Powerstroke
99 E300 Turbodiesel
91 Vette with 383 motor
05 Polaris Sportsman 800 EFI
06 Polaris Sportsman 500 EFI
03 SeaDoo GTX SC Red
03 SeaDoo GTX SC Yellow
04 Tailgator 21 ft Toy Hauler
11 Harley Davidson 883 SuperLow

Only way you can tell besides anecdotal "evidence" is if you have other engines outside and they are dirty and this one is clean.

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01 Ford Excursion Powerstroke
99 E300 Turbodiesel
91 Vette with 383 motor
05 Polaris Sportsman 800 EFI
06 Polaris Sportsman 500 EFI
03 SeaDoo GTX SC Red
03 SeaDoo GTX SC Yellow
04 Tailgator 21 ft Toy Hauler
11 Harley Davidson 883 SuperLow

I am not sure the thickness of the Oil is the entire story.
Some of the V Detroit Diesels have a similar Oil Pump on the front Crank end and the sometimes crack. They were designed to use 30-40 wt Oil when the Engines first came out.

I am guessing that that Crankshaft vibration, flexing stresses and thrust also are effect the Oil Pump.

In any event when I was doing a partial rebuild the Parts Guy told me about the issue and sure enough the Oil Pump was cracked and that is what was keeping the Engine from Rotating.

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84 300D, 82 Volvo 244Gl Diesel