It is the additives for the coolant. SCA stands for "Supplemental Coolant Additives " and you need to keep the levels right to protect cavitation problems. Most people don't bother with this, sad to say. At 150K+ I would do a complete flush and replace with a true ELC ( extended life coolant) I use the Fleetrite ELC, no better coolant out there, good for 300K and out to 600K if the extender is used at 300K with no checking of SCA's needed. Just do a proper flush, PM for directions. I would also install a coolant filter while doing this. It will save your WP, I would also flush your brake fluid as well as Power steering fluid. The PS and TC take ATF and I would use synthetic, it does help. Mercon V is okay in these two places. However if you have a auto trans only use Mercon/Dexron III fluid, this was discontinued in June of 2007, so once the supply is gone it's gone. Some other things to do would be the Zoodad mod, and the CCV mod is a must for every 7.3. Here is a link.