Depends on application for me. I have a collection of 70's vehicles 4 79 Broncos, 77 Vette, 72 Cheyenne etc, that get 15w40, 10w40, 20w50 depending on what I have. Most of these get oil changes 3-5 yrs. 2 Broncos have swapped 460's, the other two 400's, obviously all flat tappet. Again, those get HDEO Delvac 15w40 or BradPenn 20w50 or 10w40. None of these are driven/started when its cold, all toys. 96 K1500 (another garage queen), bought from original owner I worked with in 99 with 28K. 91K on it now. I use M1 or M1 EP 5w30 and change every 5K, usually about 4-5 yrs. 1992 F250 460 270K+ orig engine. 10w40 Pennz HM right now, but 15w40 most common, it sees occasional use but heavy towing. 95 K1500 260K+ on orig engine, had bulk 10w30 most all its life (bought it from a Toyota mechanic at 255K). 4 Chevy Cobalts, bought 05 new now has 265K on it, no issues, has had mix of syn and conv 5w30 all its life, with a 10w40, 15w40 etc mixed in occasionally if thats what was on hand when I changed. Basically, to make a long story even longer, anything daily driven/started in cold weather gets 5w30 mostly, 10w30 some. All old school flat tappet stuff gets "thick". None of those likely to be cold started (maybe 92 F250), but even then driven carefully by ME, not a wife or teenager who drops in gear before idle slows at all. Boat has 502 Mag, gets BradPenn 20w50. almost 500 hrs runs good as it did in 95. Wifes 11 Equinox had only M1 5w30 (Dexos), almost 60K (no issues), but all Ive read on GDI engines I MAY start using Delvac 5w40.