It depends on the oil. Mobil 1 5w-30 is thinner than M1 10w-30 at any temp below 100*C. At 40*C (104*F) the 10w-30 is 14% thicker than the 5w-30. At 0*C the 10w-30 is about 20% thicker. At -25*C (-13*F), it's about 40% thicker, and it's cold cranking viscosity is about 70% higher than the 5w-30.
Mobil will send you a temp vs visc chart if you ask for VTA018A.pdf. It's real handy for figuring this stuff out.
[ November 24, 2002, 01:28 AM: Message edited by: Jay ]