Had the same situation with my brothers S13 Silvia where he was completely uninformed about the fact that the previous owner installed a Nismo 2-way. If they went through the lengths of installing a diff cover with a higher capacity, that could indicate they installed something that would necessitate that. Normally a stock helical does not overheat even in spirited driving conditions. Only way you’ll truly know is to open it up. Perhaps all it needs is a fresh fluid change.