ORIGINAL: Jim Thomerson
One of the engine manufacturers once remarked that 3 in 1 oil kept him in business (sorry, I don't recall which one). 3 in 1 is a penetrating oil with a lot of solvents. When these evaporate, a fairly hard gunk is left behind. The same is true of Marvel Mystery Oil. Either is fine if you are going flying next week. I use Rislone and have had engines stay free for years. If you run some castor oil in your fuel, there is less need for afterrun oil.
Jim
3 in one oil is a mix of oils no solvents, see the MSDS for three in one oil.
http://www.wd40.com/Brands/pdfs/msds...purpose.us.pdf
Now compare that to WD - 40 made by the same company.
http://www.wd40.com/Brands/pdfs/msds...aerosol.us.pdf
Notice the distillates in the WD-40 that is not in WD-40? IMO 3 in One oil is better than MMO, but only good for long term storage. The only good after run oil is castor oil.