hello marin and other sages- a frequent and probably the oldest question revisited and asked on auto forums. have wrestled with it since 65 when top recommended oil was sae 30 weight high detergency with various additives. valvoline and castrol were top recommended names. multi weight oils were just getting popular. now single weight oils are frowned upon due to slow flow in colder temps and alleged lower mpg due to slow flow. good point about old engines like the 240 using dino oil. more than one sage has told me synthetic oil causes leaks when you switch in old engines used to dino. havent used them . one respected cheapskate wrencher tells me synthetic is ok and better if you use it from the beginning in new cars. this seems like accurate advice since he has done that with 2 new camrys each doing 300k with no engine problems. as one who has read about, wrenched and cussed tanks since before the tet offensive what do i do. most of those years i have used a top rated 10-30 quaker dino, purolator oil filters and regular oil changes(4-5k). current fleet includes a 92 245, 01 saturn and 01 merc marquis. all well past 100k and engines run well. do use 5- 20 in the merc b/c the book insists on it. the volvo engine does stand out though; never add oil between changes, good mpg 25/30,no leaks and seems tight and strong. volvo book calls for 10-30. dont see much single weight oil any more. think that is relegated to racing flatheads on dirt tracks. feel free to criticize my analysis. thanks tons oldduke