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The GCF of 30w and 75w is 15w.
15w means they are the viscosity (flow) when the oil is cold, the -30 and -40 is the viscosity of the oil when hot. Therefore the 15w-30 being the lower Viscosity means it will flow fast then the 15w-40 when the oil is hot.
If you mean: 15w -2(-w+12) then it is 17w -24 when simplified
are you sure you don't mean MINUS 5? 15w^2 - 10w - 5 = ( 3w + 1 ) (5w - 5)
15W-40 15W-40
w = 13 add 28 to 180, then subtract 13 from 180 to balance equation. Then 4w + 11 w = 195. Add 4w + 11w =15w, so 15w=180, divide 180 by 15, w = 13 4(13) -28 + 11(13) + 13 = 180
15w 40 is recommended for this size engine. 10w 40 will suffice if no 15w is available.
Really depends on environment. 15W-40 is common as an "all around" oil for diesel engines, while 5W-40 might be more common in particularly cold environments.
15w 40 engine oil can be used for all temperatures down to -15 deg centergrade !
The weight of the oil when it is cold. The 15w is a thicker oil than the 10w when it is cold.
15w-40