In the last couple of years I started using single grade engine oils during the spring, summer and early fall months. Because of the cold Canadian winters, I use synthetic or group III 0W30, 0W40 or 5W40s for cold start lubricant performance. HDMO 15W40s and about a case a year of HDMO 10W30 are about the only dino multi grades I still use quite a bit of and are usually in vehicles that only see occasional use, or may be pressed into use at any time of the year. Last month I put 4000 miles on my Windstar van ( 95 3.8 120,000 miles)with SAE 40 in the crank case. I'll run that oil for another month or 6 weeks and change it at around 6000 miles, likely to 5W40. I have absolutly zero consumption between changes so I'm not worried about the high ash HDMOs hurting the cat-con. Previous to switching to xxW40s and single grades, the vehicle used about a litre of oil every 3500 miles, more in the hot months with 5W30 group II PCMOs, not brand specific. I have aproximatly 10 vehicles 94-2003, not all mine, but maintained by moi, with similar findings. Zero oil consumption must mean longer cat-con life. The more viscous lubricants may hurt at the pumps a little, but drivers have more of an influence on economy than engine oils ever will. The use of VI improvers are a way down in my shop. My question is, does anyone else use single grade motor oils as opposed to multi-grades in warm climates or during the summer months?