Ian - The "SM, SL, SJ, etc" refer to the Society of Automotive Engineers rating system for oils. In this case, gasoline engine oil and deisel oils for use in vehicles with catalytic convertors. The lower the letter, the more modern the blend. The EPA forced manufacturers to lower the additive packages containing the zink because of pollution issues associated with catalytic convertor failure. SM has less than SL, SL has less than SJ. Shell rotella and any deisel oil that carries the SAE gold medallion has the same zink limits as the gasoline engine. So if you can find older stock oils and check the SAE medallion that says SJ you're fine. but stocks are very hard to find that old. Check local parts stores that dont move volumes of oil. I bought 3 cases of SJ 20-50 last year when I found some in Car Quests warehouse system. But if the bottle says SM, it's got about 30% the zink it used to. The up side is first, the XE268 cam is a fairly mild grind and if you have the Comp 901 springs you should be fine. Second, as this is a "fun" car, you can also buy oil sold as "not for highway use" that does have the higher zink. Brad Penn, Valvoline racing (the not for street stuff, not SAE rated VR-1), Joe Gobbs all offer oils like that. They are not synthetic, and they are more expensive and usually dyed so they are mixed up with std oils. I recommend straight 30wt off road VR1 with most of my engines. It can be bought easilly and has plenty of zink. If you're changing oil twice a year it's not a big deal to spend $50 per oil change on oil.
edit: There is no oil between the lifter and cam lobe at startup At least not enough to form a wedge to keep the parts from touching. That is where the ZDDP works. Cams are only splash oiled... no pressure there. So the valve spring forces the oil out from between the parts. Rod bearings on cylinders with pressure in them have the same issue. That is where "cold start" (a misnomer... all engines hot of cold that are turned off suffer from this) wear takes place. The faster an engine fires the less wear it has as a result. I dont believe it will go away. But the single largest friction spot in the old engines is the lifter face on startup.