There are two questions that always provoke curiosity when I see them.
One is "Will .38 Special +P hurt my .357 Magnum?"
The other is "Will Speer shotshells hurt my barrel?"
In the first, I've never heard of cracked Trooper .357 Mag frames from using .357 Mags (although I suppose it's possible), much less the much lower-powered .38 +P.
What tends to go first on the older V-Spring Colt DA revolvers is the hand, not the frame, and that certainly will take longer with a .38 than a .357 Mag diet.
In the second, if those shotshells damaged bores, Speer would pretty much not still be selling them continuously for over 40 years.
You're right in older revolvers being made different than today, both in terms of design & metallurgy (depending on how old), and that the original Troopers used entirely different actions than the MKIIIs.
Your '68 Trooper .357 (and my '66 Trooper .357) won't hold up to .357s as long or as well as the MKIIIs that followed in 1969, but unless you get a little loony in handloading the timing will go before the frame does.
Shooting any commercial .38 Special load from a reputable manufacturer will put far less stress on the gun than a .357 Magnum load.
Wear also depends on how much you shoot.
500 rounds a week in either caliber will wear out an older Trooper fairly quickly.
Something to remember is that those guns were designed in an era where only target shooters shot them regularly, and they had to be rebuilt periodically with heavy use back then.
Putting 20,000 rounds through a gun simply wasn't envisioned for the average buyer. That only came along within the past 30 years or so.
Many people today expect far more from those guns than their designers ever did.
Follow RC's advice on the carbon rings, take it easy on the old Colt (in terms of volume, not caliber of ammunition), and keep in mind that Colt won't be servicing these things forever.
The day will come when they'll stop, and at that point you may not be able to keep it going if you wear it out.
Gunsmiths capable of working on the actions are down to a handfull nationwide, and parts are drying up.
Denis