What is the preferred naming convention for Func method parameters?

There are precedents for using a noun in the Framework, e.g.

Enumerable.Average<TSource>(this IEnumerable<TSource> source, Func<TSource, decimal?> selector)

Enumerable.Count<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)

Enumerable.GroupBy<TSource, TKey, TElement>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector)

ConcurrentDictionary<TKey,TValue>.GetOrAdd(TKey key, 
            Func<TKey, TValue> valueFactory);

The noun is often an appropriate verb with an agentive suffix.

In your example I would use something like loader or possibly valueFactory. I personally don’t like cacheLoader because presumably it’s the caller rather than the delegate that does the work of inserting in the cache.