While this subject "OIL" has been posted to death here, one more isn't gonna kill ya!

I have just one question, and hopefully only one answer!

Question: Why do I see oil sold at retailers selling almost exclusively 5-20, 5-30, 5-40, (the emphasis is on the 5) what happened to 10 and 15 weights?

I read a thread in another MB forum a while ago from an older mechanic saying that you should not put a 5 weight oil in older engines, that from his life's experience as a mechanic, an older engine will age faster using light oils.

After reading his reasons why, chiefly that bearings have more space between and light oils just don't lube as well in these cases, I hesitate to use any oils lower than 10, and even at that, I feel safer with 15 wt.

Any thoughts?