Googled the following
"When referencing motor oil weight, you are actually referring to the viscosity grade or the tendency of a liquid to flow slowly or quickly.
In regard to multigrade oils, the first number in the code, such as 10W in SAE 10W-30, means that the oil can still be pumped by the engine at a temperature as low as a single grade 10W oil. A 5W, like in SAE 5W-30, can be pumped at an even lower temperature, and a 0W, like in SAE 0W-30, will pump at the lowest tested temperatures. The W in the designation is commonly thought to represent weight, but in fact stands for Winter. The second number, 30, indicates how well the oil will flow when heated to 100C/212F or higher.
As engines have become more technologically advanced, engine clearance levels and viscosity grade recommendations may have been reduced, hence the reason your father swore by SAE 20W-50 and your engine calls for SAE 5W-30. If a thicker oil is used in some of todays high-tech, small clearance engines, oil pressure may increase, but the possibility of improper lubrication can result if the oil can not adequately flow through the engine."
so I guess the 10w 30 is slightly thinner at normal operating temps than the original specified 10w 40