I would say the 15W-40 is just too much for it, if I remember correctly you have a 292? I run 10W-30 in my 283 with no issues in the cold, and in hot weather it holds 40 psi wound out.

1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy