Originally Posted By: VQLT
66 barracuda holds approx 5 quarts
mid-90's Saab turbo 4 quarts (specs 10-40 or 15-40 if temp consistently above 60 degrees
Does either vehicle allow a 30 weight?
What engine in the 'Cuda? Is it built or stock(ish)?
Reason I ask is that oils before [insert correct date] sheared much more than modern oils. A 1960s 10w40 might have sheared down quite a bit over 3-4000 miles, and your modern 10w30 will probably stay in grade. Another example is that 10w30 in the late 90s didn't shear but 5w30 did, so 10w30 was recommended then over 5w30 for warmer temperatures.
If that's not a comfortable response, then go with mixing a 50 grade with the 30 to approximate a 40.