A tiny, On Topic rant...
Oil Additives is NOT The Answer...
The GM Atlas Vortec 4, 5 and 6 Cylinder Engines are unique in their need for the 5W-30 Oil Viscosity due to the design of their Helical Cam Phaser constantly being adjusted by variable oil pressure governed by the CPAS with a 0-25 Degree range of freedom. Yours fits this category as an Early Model design; perhaps less so for the later years.
This makes these
VVT Engines subject to being ambushed by the improper use of Higher-Thicker viscosity oils ...
and often Dirtier Motor Oil than should be allowed that will clog up the S/S Screens on the CPAS and cause the Engine to stumble in various RPM Ranges as a result.
The importance of having MORE regular Oil Changes NOT less as an Engine ages comes into sharp focus when you consider that it is the Well - Worn. Leftover Motor Oil down inside the Crankcase that must Absorb, Combine with, Dissolve and Hold Onto all of the Un-Burned Gas Blow-By Residue and the resulting Black Carbon Mung Build Up.
Lastly, an enormous amount of Gasoline Fumes will also leech out of the Old Motor Oil over time into the PCV and this can skew the A/F Balance as this Unmetered Fuel enters the Air Stream through the PCV Tube into the Resonator, Throttle Body and right back inside Engine.
This can make for a situation where the O2 Sensors cannot figure out a proper STFT & LTFT on many High Mileage Motors after inhaling Gasoline Fumes in addition to EFI contributions to each Cylinder.
If you think I'm crazy...
Pull out the Dipstick and smell the end of it. The presence of a Heavy Fuel Scent should encourage people to eschew trying to "Wring Out Every Last Mile" just to avoid the inconvenience of having to Change the Motor Oil.
Take pity on an Engine that has given such an amazing amount of service and thus, does not deserve such ill treatment.
THIS Video will Demonstrate just How BAD This Problem can GET as an Engine in DESPERATE NEED of a Berryman's Chem-Dip Soaking for around Four Hours before Re-Assembly: