A G200 EXPERIENCED HI OIL TEMP EICAS WARNING AND OIL SMOKE IN THE COCKPIT ON CLBOUT. THE FLT CREW RETURNED TO THEIR DEPARTURE AIRPORT.

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DEPARTED ZZZ AT XA45. THE PIC HAD JUST PRIOR TO START-UP DEFERRED A L AHRS PLATFORM AND INFORMED ME THAT THIS WAS DONE LEGALLY AND CORRECTLY. HE FAILED TO INFORM ME THAT WE HAD A VMC ONLY REQUIREMENT WITH THIS MEL. I WAS THE PF AS L SIDE EFIS DISPLAY HAD NO COMPASS/HDG/PITCH INFO. TKOF AND CLB NORMAL BUT AT 8000 FT RECEIVED A 'L OIL TEMP HI' CAUTION EICAS. PERFORMED QRH; REQUIRED MONITORING OF TEMPS WHICH STABILIZED AND CONTINUED FLT. WE OBVIOUSLY ENTERED PCA ABOVE FL180 WITHOUT REQUIRED EQUIP FOR IFR FLT. ON CLB FROM FL260 TO FL330 IT OCCURRED TO ME WE COULD NOT FLY IN RVSM AIRSPACE AS OUR AUTOPLT WAS NOT ENGAGING. WE NOTIFIED ATC ON CLB; AND AFTER RECEIVING SEVERAL 'STAND BY' AND REQUEST FOR FLT BELOW RVSM AIRSPACE; WE WERE RECLRED TO FL270. WE OBVIOUSLY HAD ENTERED RVSM AIRSPACE UP TO AROUND FL310 BEFORE DSCNT. WE THEN LANDED NORMALLY AT ZZZ1 TO PICK UP ADDITIONAL PAX FOR CONTINUATION OF OUR FLT TO ZZZ2. THE PIC WAS ON PHONE TO MAINT BUT FAILED TO WRITE UP THE OIL TEMP ISSUE. TKOF WAS NORMAL BUT AT APPROX 10000 FT RECEIVED ANOTHER 'L OIL TEMP HIGH' EICAS AND ALSO OILY PACK SMOKE ENTERING CABIN AND COCKPIT. I DONNED MASK; REQUESTED LOWER; 180 DEG RETURN TO ARPT; AND QRH. PIC (PNF) COMPLETED THESE: PRESSURIZATION WAS DUMPED VIA 'RAM' TO CLR SMOKE AND RETURNED TO ARPT. THE PIC REFUSED TO DECLARE AN EMER; ASKED FOR RETURN TO ARPT ONLY. WE BOTH FELT THE ACFT NEEDED TO BE ON THE GND ASAP AND LANDED SAFELY ALTHOUGH AT 400 LBS OVER MAX LNDG WT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED THE SMOKE IN THE COCKPIT WAS APPARENTLY CAUSED BY THE ENGINE OIL BEING OVER SERVICED WITH THE EXCESS LEAKING OUT OF THE GEARBOX; THROUGH THE ENGINE BLEED; INTO THE PACK; AND THEN THE COCKPIT AND CABIN. THE OVER SERVICE WAS ALSO BELIEVED TO HAVE CAUSED THE HI OIL TEMP EICAS WARNING BECAUSE TOO MUCH OIL REDUCES THE AERATION COOLING OF THE OIL.