I have some HDEO 15w-40 that I want to use in my old cars but think it might be a bit viscus in the winter. Would it be OK to mix say 2 qrts. of the 15w-40 with 2 qrts of 5w-30 (or even 5w-20). The 5w-30 is regular API SM/N passenger car oil. Is there any concerns for additive package conflicts between the HD diesel rated oils and the regular (non-diesel) conventional oil? I usually run the oil b/t 5,000 to 7,000 miles.