I got me a 84 f150 xlt 5.0L c5 auto. Truck has been well taking care of. Was wanting to change its oil and saw that the previous owner was using 5w30 but to me a flat tappet 5.0L would shear it out of grade over a 10w30. Truck also runs about 3k rpm at 65mph. Should I use RGT 5w30 that I have or use rotella t4 15w40 that I also have. My 88 truck seemed quiter on 40 than 30 but had more miles and used oil alot. What yall think?