It is a little different if you are dealing with "winter" in Tucson vs. Minneapolis: a synthetic 15W-40 might be a good year-round oil for almost all the lower 48. On the other hand, an HDEO in 10W-30 (say Chevron Delo 400 in this grade) would most probably do the same for colder climates.

My personal belief is that MOST 5W-30 oils are a little stressed in '91 era gas motors, the obvious exception being Amsoil's excellent HD 5W-30 oil which, though pretty expensive, could be used, IMHO, year-round and with reasonable extended drain intervals almost anywhere.

Cheers!

p.s. I use Amsoil AME 15W-40 year-round in my '91 420SEL.

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'91 420 SEL @ 199K, '92 SVX @ 181K, '93 SC400 @ 86K, '93 Kaw ZX-11 @ 30K, '87 F250 @ 181K , 2001 Valkyrie Interstate @ 6K, Y2K Honda NightHawk 250 with 1.5K, '88 420SEL I.@ 179K & the 2nd latest, an '88 420SEL II.@ 210K runnin' parts car, '85 F150 300/NP435