Recommended summer oil for a 1965 GTO - Pontiac GTO Forum
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post #1 of 4 (permalink) Old 04-10-2019, 09:25 PM Thread Starter
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Recommended summer oil for a 1965 GTO

As it's getting closer to summer I am wanting to swap out my engine oil. The car is a 1965 GTO but I am not 100% sure the engine is original but I believe it's mostly stock (Screenshots attached). Currently the car runs fine but when it's cold my oil pressure will hover around 60psi regardless of whether I am at idle or driving 70mph on the freeway. If I go beyond 70+ it jumps up to 70psi. Once the car warms up it starts to drop back to around 30psi at idle and then increases about 10psi for every 10mph faster I go.

It can take a good 30+ minutes of driving before I see the change where the pressure starts to follow my speeding up or slowing down.
I'm also not sure that it's old school oil with Zinc as I had a shop replace the valley pan and they did an oil change with whatever shop oil they use but I am guessing 5W30 (without zinc).

I live in the San Fernando valley (Los Angeles) here in California so during the summer it can be 100deg+ on warm days and almost never drops below freezing. I believe the engine has flat-tappet cams so I am thinking I need a good oil with Zinc. What do you recommend I put in for the summer?

Engine Block Code Front: 530660 WA
Engine Block Code Back: 9778789
Intake Manifold Code: 9778816
Valve Code: 8775
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post #2 of 4 (permalink) Old 04-11-2019, 08:38 AM
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Everybody has their own idea about oil.

I live in Louisiana, which can also get hot in the summer. Don't have a street Pontiac, right now. I have Rotella T4 15w-40 in my 455 bracket engine. It supposedly still has a 1200 level of ZDDP in it. I use it because it's quite a bit cheaper than the HP brand oils, & they always have plenty at my local Wal Mart.

When I take it to the track this summer, I'll probably also add a bottle of STP oil treatment. It makes the oil a little thicker & also adds a slight amount of ZDDP.

Back in the old days, I ran 50w Vavoline racing oil, plus a can of STP. I've always experienced the same as you, having high oil pressure, 'til the oil gets hot. That's just a fact of life. Oil is thicker when cold & thinner when hot.
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post #3 of 4 (permalink) Old 04-11-2019, 01:41 PM
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I'm in hot Fresno CA, and run Rotella 15/40 in my old flat tappet cars. Have for years. Never any issues.
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post #4 of 4 (permalink) Old 04-17-2019, 11:42 PM
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I agree, everyone does have their own idea about oil. You should be fine with any name brand oil that advertises high zinc (zddp) content. Either synthetic or dino oil. I like Amsoil Z-Rod which is designed for older cars. I strongly recommend not to add STP oil treatment or any other oil additives, they just aren’t necessary if you use a good quality oil to begin with. Here’s why, years ago I was adding STP to my Road Runner simply because my engine was tired and I fell for their claims. I was planning a rebuild and figured it would by me some time. About a year and two or three oil changes later and I decided to do the rebuild. When we disassembled the engine at my friends shop (who built race engines for a living) he immediately reprimanded me for using STP oil treatment. I was shocked and asked how he knew I was using it, then he proceeded to point out this gray clay like build up in the corners and galleys throughout the block and heads. Apparently it’s most common reaction is to turn into this gray substance that separates out of the oil and settles into all available corners. He told me it was a waste of money and never use it again. Imagine what could happen over time as this stuff builds up, especially to modern engines that tend to have tighter tolerances and smaller oil return passages.
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