2002 Pantera 600EFI with Power Valves. Excessive buildup on the power valves to the point where they are sticking. Required a good cleaning at 500 miles. Had them cleaned again the other day at 1500 miles. Dealer said they were so carboned and gummed that he had difficulty getting them out for cleaning. Been using the APV oil. Is this normal for power valves? Appears that they need a cleaning every 500-1000 miles. Noticable performance drop just before the major cleaning. Anyone else experiencing the same? Can the use of other synthetic oils help this?