sbc350gearhead, I did not say that synthetic 5W30 oils were bad. I said I have not tested them so I can not coment on them. I have seen thousands of cars ran on 5W30 and all of them were worn out and tired between 120,000-150,000 miles it did not matter if they were import or domestic!! By worn out I mean burning more oil then they should, uneven compression, low compression, a lot of carboned up rings seveer taper in the bore, low oil pressure, main and rod knocking at 100,000 miles with 3000 mile oil changes etc....
I always recomended M1 10W30 and 15W50 to all of my customers and those that did this always had long life. THe shop also pushed this combination. Most places I worked pushed simalar combinations. For those customers that did not want synthetic due to cost I used to use Castrol 10W30 GTX in the winter and 20W50 GTX in the warm months.
5W30 and 0w30 are soloutions looking for a problem. Short of artic conditions these oils do not protect any better then their 10W30 counter parts. Short of artic temps they are not going to flow that much faster then 10W30. They tend not to be as stable and consistent as their 10W30 counter parts of the same chemistry. No one has run 10W30 M1 or Amsoil then run the same brand of oil in 5W30 with the same chemistry and shown any advantage under identical conditions.
To date evertone that has run a thicker oil like M1 15W50, Delvac 15W40 etc. in the warmer months has shown a huge improvment over the anemic 5W30 they ran before. It is also preety funny that the company(General Motors) that has pushed 5W30 oil the longest and hardest is also one with the most oil consuption problems under 120,000 mile LT1,LS1, North Star, Saturns. Companys that have jumped on board are now haveing pronounced problems with oil burning and gelling in engines that traditionaly did not have that much of a problem with either issue(Toyota). BMW had issues with 5W30 synthetic in their engines.
Just about everyone on this site that has oil burning issues under 150,000 miles has been runing a diet of mostly 5W30! I say run a good 10W30 conventional or synthetic and a good 5W30 synthetic if you live in artic like temps in the winter. Then run 15W50 M1 in the warm months and skip all the oil burning and premature wear.
Their is enough evidence all around that it should not be a difficult leap to make! I am currently trying 5W40 to see if I can find a truly year round oil that will give the same type of life as my old combo of 15W50 warm weather 10W30 cold.
Personely I think it is a combination of ash due to volitile oil wearing out the valve guides and carbon deposits loading up the rings. I have no intention of testing 5W30 since it does not solve any need that I have yet.