How far do you want to go with the recycle process simple cleaning and rejuvenation to be use where non detergent oils would be acceptable
Or do you want to turn it into diesel to be used in any older diesel engine with a mechanical injector pump
For both processes you start with a good filtration system then to a centrifugal filter one run by air is preferable and at least capable of spinning fast enough to remove down to .5 micron from there you need a heat source such as that found in an electric hot water heater this is to evaporate any solvents such as petrol antifreeze water and others. from there another centrifuge at this stage you now will have usable clean motor oil without any additives or detergents
If you want to make diesel then you will need to add in a catalyst in the heating stage and must go to a higher heat and monitor the PH level closely. there are several do's and don'ts on utube many though do not adhere to all safety precautions .
This is one reason that I only gave generalities of the process I wouldn't want someone to build one of these off of my method alone, since to explain all of the safety precautions required will require far more in depth conversations that are not easily just put into words it takes hands on experiences with someone who has done it successfully for years and I have only done it sparingly just to see if I could