Putting oil on O rings
 

Do you always put a light film of oil on an O ring, no matter what part you are working on? Thanks.

Hugh Sr
1984 190d (W201)

They should have a film of whatever fluid they are going to seal on them. Transmission O-rings should have ATF on them.
Brake O-rings should have Brake Fluid on them.
If it is something using Grease, Grease the O-rings.
And, so on.
The exception would be if you have some special O-ring Lube than can be used as the have for Brake Calipers.
The one place I worked during Fuel Injection Pumpy assembly we use a mix of 50% Motor Oil and 50% STP.

I usually put some waxy/silicone grease on any o-ring I install. Don't know if it's wrong or right; I do it to lessen the chance of it rolling out, and possibly tearing or pinching and becomming dislodged. I think the grease I'm using is safe for this application - that is; not degrading the o-ring. My 2 cents; any valid counterpoints will be considered and would be appreciated.

And AC o rings would have Nylog on them of the correct color.

For the amount of film that will be applied to the O-ring, almost anything will suffice. For example, using 15W50 oil on a transmission O-ring: what would happen to your tranny if you put a couple drops of 15W50 into the reservoir? And I'm being generous when I say a couple drops, because you're not going to expose much surface area for the O-ring to present foreign lubricant to the system. There are silicone O-ring lubricants that are used in military gear. For the small amount that gets into the system, it would have to be a highly critical application to make a difference. Myself, I will generally use the same oil/lubricant that I'm sealing against. For non-lube apps (air/water), I use silicone.

I think mostly for O Rings they are lubricated to make sure they do not stick in some place and get crimped... so they will slide to the correct position for their application.

Yes also so they are less likely to stay twisted . It prevents cutting on install as you stated . As 911 stated try to stay with what the oring is going to be used in especially when it comes to brakes and Ac orings . Brake fluid on the wrong type oring will turn it into a sticky goo.

The motor oil and stp works good I use it or lucas oil additive when assembling cylinders. It prevents the seals and orings from cutting going across the inlet ports.