Changing to syn. can trigger temporary oil leaks, especially at the F&R main seals, until the seals become conditioned to the new oil. So check the level FREQUENTLY after changing, and top-up as needed. I didn't on my '94 CV, and the engine seized at 70mph ~1,000 miles after I changed. That's why it has a '00 engine now.
But on these old smallblocks, you won't see enough improvement to justify the cost of synthetic oil (which typically contains NO synthetic oil anyway). IMO, that money would be better spent on a good professional engine flush (I recommend BG products), or intake system cleaning (again, the BG 3-part process). Call a few local shops to find out which ones use BG products.
If you want to read more about why "full 100% synthetic oil" is made of 100% dino oil, check BITOG
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