So, what is the difference between 15W-50 vs. 10W-40 engine oils? A 15W-50 is a thicker oil better suited for high-performance engines in hotter climates.
The API standard uses weight. A 20wt single weight oil will have a specified viscosity range at 100C and a much thicker specified viscosity range at 10C. A 10W-40 multi-weight oil will be a 10 wt oil that has viscosity modifiers added to it to cause it to thin less than expected, as the temperature increases.
Rotella is a North American brand. Rimula is their big brand from Europe. Shell US introduced a cheaper, lower performance (yet decent) CI-4 Plus alternative to Rotella here in the US a few years ago. The performance difference between the two was easily seen comparing CI-4 Plus formulations (though Shell is on it's 3rd CI-4 Plus Rimula
Mystik ® JT-8 ® Synthetic Blend Super Heavy Duty Engine Oil SAE 10W-40. View Details. Mystik ® JT-8 ® Synthetic Blend Super Heavy Duty Engine Oil SAE 15W-50.
I recently put Delvac Super 1300 15W-40 in my LT4 Corvette with 45,000 miles on it. Engine runs great and doesn't burn oil between changes. The dipstick level didn't change noticably in 6,000 miles on M1 5W-30. It appeared to drop less than 1/10 of the way to the add mark. The 15W-40 is in there for a rinse cycle on ARx. . 255 146 185 482 277 271 208 90

difference between 10w and 15w oil