Based on the mileage of his motor that guy is accounting for the wear and tear on the motor. You are correct with the climate and such so if you are in Canada or northern us, I would go with lesser viscous oil. But how about you send him to any oil change place or dealer and get their opinion. Dollars to dimes I bet they tell you what I said.
Also, if it was an lb7 with 70k on the clock I would use 5w in it but since it has 170k I am going to play it safe and say 15w.
Oil weight refers to two connotations when viewing the rating. Just run over to Mobile or pens oil web site and look at how to read an oil rating. Because that's what it is, a rating to determine the appropriate time to use it.
2004 Silverado LT LB7
Moded air box with custom 5 inch Steel tubing, Full EGR delete with fed up pipes, IAH Delete, fed y bridge, PPE Turbo Horn, shortened drive belt, LBZ CP3 running LB7 FPR, port/polish on IHI with rebuild, Batmo Wheel, PPE boost valve, 3.5 inch down pipe, Full turbo back 5 inch exhaust, Airdog 2 165, red head power steering gearbox, Tie Rod Sleeves, Cognito control Arms, Cognito pitman and idler arms, EFI Cory Willis Tunes, all aluminum transfer case, 33x12.5 nitro a/t on 20 inch Rhino lined American Racing Rims, 4 pair high discharge LED lights behind grill and under trailer Hitch, Rhino Lined bull bar, Bushwacker pocket style riveted fender flares painted to match, Triple Glowshift Gauge A-pillar with Boost/EGR/Low side fuel pressure @ lifter pump, LG Tablet by Verizon on flex pedestal running Torque Pro for real time logging/detailed measurement in realtime/gps speed logging, low and hi beam 4300k HID, pump rub kit, aluminum transfer case, @ 325k miles