My 95 Legacy recently rolled over 300k and im about due for a oil change also trans/diff. I have changed oil with 15-40w last couple times and added some Marvel mystery oil. I went with 15-40w due to the high miles with no issues & before I used 10-40w no problems. My question is should I stick with the 15-40w dino oil or should I change viscosity and/or to a full synthetic/blend? I have read a few guys use the 15-40w in there cars and a shop my friend worked at would use 15-40w in all the older EJ22's & 25's that went thru. But I've recently read that 15-40w doesn't lube as well as thinner oils & that the cleaners in it can be harmful to non-diesel engines.
Also what are your idea's about adding a little 80-140w to the 75-90w(80-90w) for the trans/diff to help cushion older parts?Or Is there a good additive to use for this?