Getting ready to change def. oil got 5 qt oil one happened to be 90/140w. outhers were 75/90w Will it hurt to mix.

 

I've had to mix them in a pinch....no problems encountered.

 

Won't hurt, but I think it takes 6+ qts to do 2 axles and the rear is just under 4qts, so you either got enough to do one axle with the same oil or need to go to the store anyway.

 

I've been told by lube professionals that mixing weights is like mixing different size marbles. The big ones will take all the load but I don't know if that applies to multivis lubes beings the vis changes anyway. I would also like to know this answer. One old lube saleman/friend told me the racers used to mix their own all the time before there were any multivis oils. Craig

 

I worked in an oil refinery and discussed this with the engineers. They said it can change the VIS, but if the need is in the range the blend is at that the mixture ends up at, there shouldn't be an issue.

The issue can happen from the chemicals in the oil. They have a specific blend of chemicals in each oil / grades. This can be within the same products from the same refiner blending requirements. He stated it is best to have the same weight and blend of oil when changing, and don't recommend mixing if possible.

When changing, if you want to change manufacture, weights or VIS, do it, but do right. Like changing motor oil. When changing, if you use Rotella 15w-40 and change all the oil with, say, Mobile 1 5w-40, change it all, but stay in the requirements recommended by the vehicle manufacture.

Mixing was highly NOT recommended.

 

When I first changed the oil put in fully syn. 75/90 Moble 1 going back with same brand and fully syn.but 1 qt. 90/140.

 

not a problem, the additives in both are the same. as stated above the viscosity will be the blend ratio.