I own a VZ Commodore Executive with the 175 alloytec engine and the oil that Holden has been using at oil changes I believe is 10W30, which is probably the minimum weight you can use. According to the handbbok, if an oil of lower viscosity is used your oil consumption will go up. John Lee
Look at your oil cap. The weight is on the cap.
SAE 10w30.
The operator's manual for a 3020 should state which weights of oil should be used in specific temperatures. Generally a 30# oil should be used in the summer and a 10# oil in the winter. A Multi-weight oil such as 15W-40 can also be used in all seasons.
5W30
5w30
15/40
30 or 40 weight oil should be used.
The correct weight which should be 5w20 is listed in your owner's manaul.
SAE 20w50 100% synthetic oil.
10w-30
The manufacturer recommends that you use 10W-30 weight oil. 10 W-40 weight oil can be used in high mileage engines.
The manufacturer recommended 10W-30 weight oil in the 1979 Chevrolet Corvette. A heavier weight oil can be used in and older engine.