The turbo compressors were not that big on the B707. Installed in the front of the pylons 2/3/4.
There is NO bleed taken from the 'hot' end. High pressure bleed will be taken from the back end and/or mid way along the compressor depending on the engine type. However this air can on occasion get contaminated with engine oil (overfilling in some cases or an oil seal problem).
The air conditioning Air Cycle Machines these days all have 'air bearings'.
@Sampan Angkasa
I would say that is an APU fault especially in the descent on the 777.
The APU can have a problem with oil seals when its got a few hours 'under its belt'. When the T/L's are retarded at TOD it causes a partial vacuum in the APU pneumatic duct and any oil residue will be drawn down into the pack heat exchangers. Hence it's noticeable in the descent!