my choice is delo 15-40,it is a good split between 10-30 and 20-50.it is used in 70 to 80 % of semi trucks and has proven track record.engines in the 40`s and 50`s tend to (smudge up)and the delo has additives for this, and it will rival the synthetics in performance.valvoline is also a good choice,as is castrol.most oils on the market are suffient,some are a little better than others. the synthetics will do a little better job but i dont think the price difference warrants it if you keep it changed.