95 Z28 Pinging

My 95 Z, 58k miles is pinging on acceleration when going up a grade on the freeway, especially when a/c is on and/or when I am carrying a passenger and
on warm days 85+ degrees.
The pinging was reduced considerably after a recent oil change, however when I took it in to the dealer shop, they put it on the computer, everything was perfect they told me. They test drove it after the oil change and it did not ping for them, same 'ol story. They told me if it happens again to bring it right in for a test drive.
I asked them to check the knock sensor, but has they said the computer did not show anything wrong, everthing was up to spec.
Is it normal to hear some pinging before the knock sensor kicks in?
Any ideas?
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What kind of gas are you using? My 94 will do that once in a while if I lug the motor on a hill when its hot out.

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I am using Chevron and/or Union 76, 89 octane premium gas.

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Where I live in California, Premium is 91 octane. My 96 Z28 runs fine on 91. Even going uphill with the AC on when it's 105 degrees outside.

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My mistake, you're right, our premium gas here is 91. I believe though a few years ago it was lowered from 92.

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What part of CA? Here in Sac - and in all outlying areas I've been to - premium is 91 (wish we had 93-94), plus is 89.
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What part of Sac are you in? I live in Newcastle, but I work in Sac. - Gary

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North Sac, between Rio Linda and Antelope (Elverta).
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your best case scenerio is the timing is off, but it sounds like the motor might be knocking on heavens door. how many miles did you go in between oil changes?
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It went about 4,000 miles between the last oil change.

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