I have been running Valvoline 10w30 conventional in our slant 6s and my older (92 and 78) 318 engines for years..... Seems now that straight 10w30 in Valvoline at least is selling at $28 for 5 quarts but the "max life synthetic blend for high mileage engines" in the same weight can be had for $17 for 5 quarts. Any thoughts on using this? The 92 is high mileage at 125k miles....one slant 6 is in the 20k mile range (rebuilt several years back) and the other supposedly has less than 40k but has never been opened..... Worried that the synthetic will make any leaks we now have MORE prevalent.... But to save $10 a jug (multiplied by 5 or 6 jugs) is attractive..... Thoughts?