My 240 D, which has been running great on well filtered WVO for about 80,000 miles, still runs fine, but for some odd reason, the temp gauge has a hard time getting up to 80C. I first noticed this on an 85 deg F day. It has forever run at 80 C or a bit above on such a day. On a cooler day about 70F, it was running at more like 60 or 70 C. Never had this before. Always runs at or a bit above 80C. Any ideas? Could the gauge or sending sensor be failing?