What Oil Viscosity for Southern California Car?

F

frwgt350net

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Feb 28, 2026

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Yesterday at 5:41 PM

#1

• Yesterday at 5:41 PM

• #1

Looking for opinions and or professional recommendations for oil viscosity for a 1986 GT 5.0 with 85k miles. Rock on and many thanks in advance.

 

Bullitt347

I have been doing it wrong this whole time

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Yesterday at 6:04 PM

#2

• Yesterday at 6:04 PM

• #2

Stock was 10W-30 I believe.
That should be sufficient.

 

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AeroCoupe

lube between the nut and the face. I know my lubes

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Yesterday at 7:08 PM

#3

• Yesterday at 7:08 PM

• #3

Any mods and where do you live? Location can have an impact on viscosity. I run 10W-40 as it gets pretty hot in Oklahoma and the car never sees the road from about mid-November through mid-March.

 

F

frwgt350net

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California

Yesterday at 7:12 PM

#4

• Yesterday at 7:12 PM

• #4

AeroCoupe said:

Any mods and where do you live? Location can have an impact on viscosity. I run 10W-40 as it gets pretty hot in Oklahoma and the car never sees the road from about mid-November through mid-March.

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Zero mods - I'm in Southern California and for the past three weeks its been slightly over a 100 dees grees - average year around temp here I would say in the mid 70's 80's

 

AeroCoupe

lube between the nut and the face. I know my lubes

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Yesterday at 7:34 PM

#5

• Yesterday at 7:34 PM

• #5

I would go with what Bullitt347 suggested then. If you have good oil pressure then no reason to do anything else. If you have cats then stay away from lubricants with zinc, phosphorus, and sulfur as they have been shown to kill converters.

 

F

frwgt350net

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Feb 28, 2026

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Yesterday at 7:44 PM

#6

• Yesterday at 7:44 PM

• #6

AeroCoupe said:

I would go with what Bullitt347 suggested then. If you have good oil pressure then no reason to do anything else. If you have cats then stay away from lubricants with zinc, phosphorus, and sulfur as they have been shown to kill converters.

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I've been runing that Rotella 15-40 diesel oil, this time and last time. I gues on the next go around iI'll go with 10-40 but what brand would you consider.

 

AeroCoupe

lube between the nut and the face. I know my lubes

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Yesterday at 7:48 PM

#7

• Yesterday at 7:48 PM

• #7

Man I do not run oil that you would run so I am of no use there. I run Brad Penn with all the additives being I live in an emissions hospitable state. You need some of the SoCal guys to chime in here. I updated your thread title to get the to come out from under their rocks....

 

General karthief

wonder how much it would cost to ship you a pair

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Yesterday at 8:12 PM

#8

• Yesterday at 8:12 PM

• #8

JMO
Run a semi or full synthetic 10w40 or 30, change it around 5k miles with a good filter ie; not Fram. ..keep it running good and enjoy the ride.
Oh, I am NOT in Cali

 

Noobz347

Stangnet Facilities Maint Tech... Er... Janitor

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Yesterday at 8:17 PM

#9

• Yesterday at 8:17 PM

• #9

frwgt350net said:

I've been runing that Rotella 15-40 diesel oil, this time and last time.

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That oil will ruin your catalytic converters through working PCV and EGR systems.

 

Mcmahst

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Jan 19, 2021

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Yesterday at 8:28 PM

#10

• Yesterday at 8:28 PM

• #10

I live in so cal and I usually use Mobil 1, the weight recommended by the OEM. We have a low dust, moderate temperature, no freezes environment. It’s easy on the engine. Good luck.

 

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LILCBRA

I wish I didn't have all of these balls in the air

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Yesterday at 9:35 PM

#11

• Yesterday at 9:35 PM

• #11

As I understand, diesel engine oils aren't a good substitute for gasoline engine oils. Something to do with the detergents if I remember correctly.

But I'm with @AeroCoupe - I run 10w40 in the summer months in my older flat tappet 302. I will probably do the same in my roller engine in the event I ever get it back in the car and running again. I don't think that either option would be horrible though - but I think that 40 will give you a microscopically thicker film to protect the bearings as compared to the 30 variant.