Many car manufacturers have specified 30-weight engine oils (5W-30 & 10W-30) for normal operation.  Had multi-grade oils been available back in the 1930s and 1940s, they would have specified them rather than recommending seasonal changes with mono-grade oils.  Where 10W-30 oils were originally specified, you may safely substitute 5W-30 or 0W-30 oils as these grades are only thinner than 10W-30 when the oil is cold.

0W-30 HDEO

• Chevron Delo 400 Synthetic 0W-30 (CG-4/SJ)
• Citgo Citgard SynDurance 0W-30 (CH-4/SJ)
• Esso XD-3 Extra 0W-30 (CJ-4/SM)
• Irving IDO Universal 0W-30 (CH-4/SL)
• Petro-Canada Duron Synthetic 0W-30 (CI-4+/SM)
• Petro-Canada Duron XL Synthetic Blend 0W-30
• Shell Rotella SB 0W-30 (CH-4/SJ)

5W-30 HDEO

• Amsoil HDD 5W-30 (CI-4+/SL)
• Castrol Elixion 5W-30 (CI-4 / no S-rating)
• Esso XD-3 Extra 5W-30 (CG-4/SH)

10W-30 HDEO

• Amsoil ACD 10W-30/30 (CI-4+/SL)
• Brad Penn HDEO 10W-30 (CJ-4/SM)
• Chevron Delo 400 LE 10W-30 (CJ-4/SM)
• Citgo Citgard 600 10W-30 (CJ-4/SL)
• Citgo Mystic JT-8 Synthetic Blend 10W-30 (CJ-4/SL)
• Conoco Phillips 66 Fleet Supreme EC 10W-30 (CJ-4/SM)
• Conoco Phillips 66 Guardol ECT 10W-30 (CJ-4/SM)
• Esso XD-3 Extra 10W-30 (CJ-4/SM)
• Exxon XD-3 10W-30 (CJ-4/SM)
• Irving IDO Universal 10W-30 (CH-4/SL)
• Kendall Super-D XA 10W-30 (CJ-4/SM)
• Lubrication Engineers 8130 Monolec 10W-30 (CJ-4/SM)
• Mag 1 Fleet SAE 10W-30 (CH-4/SJ)
• Mobil Delvac 1300 Super 10W-30 (CJ-4/SM)
• Motorcraft Super Duty DEO 10W-30 (CJ-4/SM)
• Petro-Canada Duron-E 10W-30 (CJ-4/SM)
• Renewable Lubricants Bio-Synthetic HD SHP 10W30 (CI-4+/SM)
• Royal Purple 10W-30 (CI-4/SL)
• Schaeffer Supreme 7000 Synthetic Plus 10W-30 (CJ-4/SM)
• Shell Rotella SB 10W-30 (CI-4/SL)
• Shell Rotella T CI-4 10W-30 (CI-4/SL)
• Shell Rotella T CJ-4 10W-30 (CJ-4/SM)
• Valvoline All-Fleet Plus 10W-30 (CJ-4/SM)