y2k345, if you have a manual for your 90s vehicles' engines then the manual will tell you what gas rating of oil your engine requires, then cross reference that manual gas rated spec with the spec on oil you may want to use and then see if that oil rating qualifies for your engine.
Your CI-4 or CJ-4 applies to the Diesel rating, the following SJ, SL or SM after / applies to your gasoline oil rating.
A typical rating for HDEO is CJ-4/SM, diesel/gas oil rating.
A gas only rated oil usually only will be SM, SL or SJ.
A CJ-4/SM HDEO meets all the lastest requirements for older and newer engines (has less zinc to protect cats on most 2007+ diesel engines), the CI-4/SL will do the job for most engines pre 2007 and has more zinc. There is a lot of discussion in here on HDEO use is gas engines, pros and cons, for gas engines the CJ-4/SM has less zinc which some believe has less harmful effect on the cat in gas engines. CI-4/SL will do fine in engines pre 2007 if you want an HDEO in gas engine and are not bothered with the higher zinc levels. I yet to hear of a cat being destroyed by an HDEO in a gas engine yet, I for one have used high zinc (1500 ppm XD3 CI-4/SL) for years now and still have no problems with cats in new and old gas engines for years now.
I think D-roc is telling you that it is okay to use an older oil rated oil that meets your engines' specs, BUT I beleive he is telling you that for the same price you might as well get a better higher rated oil than the Shell version you speak of.
If you look around you can still find good CI-4/SL HDEOs like Co-op DMO 0-40 which meets the specs of CI-4/SL XD3 which I use now because I stocked up on it, the new CJ-4/SM XD3 will do fine too for your needs on 1990 engines too, my son's 94 olds 3.1 thrives on XD3 0-30 syn HDEO CI-4/SL, Pitzel runs 52,000 kms OCI on his 3.1 with XD3 0-30, I am not advocating that to anyone but he would tell you of his success doing it.
As D-roc said there is nothing wrong with Shell version, just do as he adviced and be sure it meets your engine's oil requirement Y2; but why not use a higher rated oil like Co-op DMO CI-4/SL with a group 1V syn basestock like XD3 syn oils or XD3 CJ-4/SM with the same group 1V basestock for about the same price as the older rated shell 0-40?
only suggesting Y2.
Hope this helps some Y2.
Cyprs