C4 - which oil?

[Nails84]

Will any 5w 30 fully synthetic be ok? Was going to use Shell Helix.

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[theseoldcars]

What year is your C4? If it's an '84, it'll have a flat-tappet cam – and, even though it's long run in, you'd probably want to pick up something with plenty of zinc in it to look after the cam and lifters. Some of the high-mileage oils will tick that box, or you can go for something a bit more upmarket.

 

[Steven Smith]

You need an oil with a high zinc/zddp content (about 1100ppm if memory serves) zinc has been reduced in modern oils because it damages catalytic converters, also modern oils have higher detergent levels which cleans away any zinc additives you might use.
The best oil for an early corvette engine is Engine Archives | Anglo American Oil Company this has all the correct zinc levels. It's available in both mineral and synthetic.
Some engine builder's will only warranty their engine's if you use this oil.

 

[Nails84]

Thanks all. I let IGS American choose the oil in the end as they did the change. They used Mobil 1 5w 30 fully synthetic. They must know what they are doing so all sorted I hope.