Many car manufacturers have specified 30-grade 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

5W-30 HDEO

10W-30 HDEO

15W-30 HDEO