No you do not have to drain the oil.
Being common, no more common on a 302 than anything else. Parts get hot and cold and break down. Some types of gaskets will leak quicker than others. Using the right type of silicone also affects things.
Just clean everything real good before you apply new silicone/gaskets. Make sure all old gasket is off. I like to emory cloth all surfaces too.
Do not over tighten cover bolts. This will also cause leaks.
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Frank
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