AQ130C: Sealing copper water pipes to/from oil cooler

What sort of lubricant or grease, if any, should be applied to new rubber seals that seal the ends of the the copper water tubes which go in and out of the oil cooler on an AQ130C engine? I removed these copper pipes in order to troubleshoot an overheating problem discussed in a previous thread.

Also, is it best to put the rubber seal into the oil cooler's end cap port where the copper tube goes and then push the copper tube in, or is it best to put the seal on the end of the copper tube and then push that into the port? Or does it even matter?

Thanks!

I put a little permatex #2 on the pipe instal the seal on the pipe and then put a little on the inside recess so the seal slides into the recess easily. Im sure its not the proper way but it works fine for me.

I seem to remember the mechanic just using a marine grease. These pipes are not under a lot of pressure? Anyway I have no problems and I think it is the rubber seal that is the important bit.