97 4.6 with rust or some type of contaminant in the radiator. Does any one know what could be doing this. It seems it happened after I changed over to synthetic motor oil 15,000 miles ago.

 

Every 45,000 miles you are supposed to change the coolant. With the engine cool, open the reservoir cap, place a hose over the lower valve on the right side of the radiator, and drain into a container. On the left lower engine block, there is a block plug. It is 17mm or 11/16", gold anodized. It is soft steel, and I recommend a six point socket. Remove the plug to drain the block. Place the cap back on the reservoir, and close the lower radiator valve. Remove the heater hose, on the right top side of the engine. It should be a grey spring style clamp. Alternately blow air into the hose, and steel tube to push the anti freeze, out of the right side of the block, thru the left block plug. Caution: Plug the tube opening that you are not blowing thru so anti freeze doesn't blow all over the engine bay. ( I used the blower port on a vacuum cleaner) Reconnect hose, and flush out engine by closing the radiator vale and installing the block plug, fill radiator reservoir, run engine briefly, but not to the point it gets hot. Open valve and block plug, to drain, repeat till water is clear. If you wish, use a coolant system cleaner at this point. For the refill after a flush I used RO/DI water avaliable from a grocery store machine. I flushed twice with this water, then drained the system down. Reinstall the block plug with a good quality pipe dope, not teflon tape. Add the recommended full strength anti freeze to the reservior. This is because the engine will hold almost two quarts of fluid with the radiator valve open, and block plug removed. top of with RO/DI water or distilled water. Your rust has nothing to do with the oil, the inhibitor died in the coolant, and the coolant is eating at the internal coolant passage parts.