What Oil Viscosity for Southern California Car?

frwgt350net · Wednesday at 5:41 PM

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 · Wednesday at 6:04 PM

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

AeroCoupe · Wednesday at 7:08 PM

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.

frwgt350net · Wednesday at 7:12 PM

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.

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 · Wednesday at 7:34 PM

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.

frwgt350net · Wednesday at 7:44 PM

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.

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 · Wednesday at 7:48 PM

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 · Wednesday at 8:12 PM

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 · Wednesday at 8:17 PM

frwgt350net said:
I've been runing that Rotella 15-40 diesel oil, this time and last time.

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

Mcmahst · Wednesday at 8:28 PM

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.

LILCBRA · Wednesday at 9:35 PM

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.

86HO5.0 · 2026-08-13T05:30:53-0500

frwgt350net said:
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.

Do NOT run diesel oil in a gasoline engine.

fasterthangas · 2026-08-13T08:59:19-0500

Simply pull out the owners manual and follow the manufacturers directions.
Don't overthink it.

AeroCoupe · 2026-08-13T09:52:49-0500

Owners manual 40+ years old and lubrication technology has advanced in 40+ years by quite a bit. I don't think it it is over thinking rather more like understanding what is available today that is better than the offerings 40+ years ago.

fasterthangas · 2026-08-13T09:57:03-0500

So the recommended 5w30 is even better today, correct? That makes the factory recommendation even more valid.

frwgt350net · 2026-08-13T09:58:35-0500

fasterthangas said:
Simply pull out the owners manual and follow the manufacturers directions.
Don't overthink it.

AeroCoupe said:
Owners manual 40+ years old and lubrication technology has advanced in 40+ years by quite a bit. I don't think it it is over thinking rather more like understanding what is available today that is better than the offerings 40+ years ago.

Thank you, thats exactly what my inner voice just said to me -

frwgt350net · 2026-08-13T10:02:51-0500

86HO5.0 said:
Do NOT run diesel oil in a gasoline engine.

I'm sure you're correct but exactly why is that -

fasterthangas · 2026-08-13T10:04:59-0500

For reference, my 11:1 363 that regularly sees 7000rpm in Phoenix heat uses 5w30 semi synthetic oil. This is the oil recommended by Driven Oil techs and my builder. Taken into consideration was ambient temps and bearing clearances.

fasterthangas · 2026-08-13T10:25:27-0500

View: https://youtu.be/HcJHJL5Ys-Y?si=6pxPalqLeSs2Pqm2

General karthief · 2026-08-13T10:36:07-0500

frwgt350net said:
I'm sure you're correct but exactly why is that -

Well first it contains additives that are bad for cats, zinc I believe is one that is bad for cats so if your running diesel engine oil and no cats you're ok. If you have cats stop using that oil, it takes a lot of miles, like many oil changes before cats show effects.
There may be other reasons not to use it but that's the one I know about
Sorry, didn't see the post from faster above.

What Oil Viscosity for Southern California Car?

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I have been doing it wrong this whole time

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

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

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

wonder how much it would cost to ship you a pair

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

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

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wonder how much it would cost to ship you a pair

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