I looked through my manuals, and couldn't find the plug gap, but it's usually in the owners manual.
I've been using Valvoline racing oil in my 4.1 (VR-1) I don't want to replace the cam again, so with the ZDDP in the racing oil, it may last a while.
For oils which have an added anti-scuff package (ZDDP) in your correct viscosity range do some research on Chevron DELO, Pennzoil LongLife, and Shell Rotella fleet and marine oils. They are available in 10W-30 and 10W-40 weights and they have about the same anti-wear package as the pre-"starburst" oils.
IMO, I would only use 15w in a 150K mi+ motor with the oil light coming on at idle(low pressure). Not a fix by any means but cheaper than a rebuild. That situation probably doesnt apply here so I'd go with the recommended weight.