what oil is safe?

What I have heard and learned about synthetic oils is that it may not be compatible with all seal/rubber compounds. I would suggest that if you are looking at building an engine look for the seals that will work with synthetic oils. I will look through some of my old school text books to try and find out what works.

After some cold starts in Dawson Creek -40 and colder, with the block heater not working, I was more that sold on the synthetic oils.