NP203, Which fluid to use? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

danadena1960 · 12-19-2003, 10:00 PM

Hey All,
My 77 has a NP203 with a Turbo 350, the question I have is which fluid should I run? I did the full time to part time four wheel conversion. Mile Marker says to use 10-40, the owners manual says to use 10-40, other falks I have talked to say to use ATF. When I dumped the fluid to do the conversion it was full of ATF. I was told to run ATF because if the seals go out between the trany and transfer the engine oil would wipe out the trany. I was also going to run full synthetic, but I was told the RTV I used to put the transfer case back together with is be eaten by the synthetic fluid. Does anybody have any input on this?
Thanks,
Dana

INTIMIBLAZER · 12-19-2003, 11:13 PM

the manual does say 10w40 but i would use dexronIIIjust because of the oil mixing problem
as far as synthetic is concerned i have no first hand experience
hope this helps,Keith

**kwmech** · 12-20-2003, 01:07 AM

Over all the years I have been working on 4xs I've seen atf, 10-40, 30 wt. and 90wt in these boxes. Doesn't seem to matter what is in them as long as there is some lubrication.

Scrub · 12-20-2003, 03:46 PM

You might get ATF to go from the tranny into the case, but I doubt anything will go from the case to the tranny. My 205 was howling a bit and I pulled the fill plug to check the level. It had at least 2 quarts more ATF than you could get into it through the filler hole. I drained it and refilled with 70-125 Gear lube. The howling stopped.
I may have done a bad there, but it is still going just fine 3000 miles later. No gear oil is in the tranny.

verdict: The ATF was comming from the tranny.

rogf4 · 12-27-2003, 01:53 AM

I would run the 10/40 or syntec. the trans will over pressure the transfer case but not vise versa The heavier oil seems to run quiter and give better lubrication

Cethern · 12-29-2003, 03:15 AM

No matter what oil you use I would go with the Synthetic. It will not eat anything up.
My Blazers and other cars I've had have used Synthetic oil for everything from the Tranny to power stearing and the diff's with no problems.

As for what Oil to put in the 203 go with what the book says. I found it weird to add ATF to my old 208. And I think there are two seals so if your vents are some what clean you should be ok.

Cethern

12-19-2003, 10:00 PM	#1
danadena1960 Registered User Join Date: Aug 2003 Location: Stockbridge, GA (Suburbs of Atlanta) Posts: 126	NP203, Which fluid to use? Hey All, My 77 has a NP203 with a Turbo 350, the question I have is which fluid should I run? I did the full time to part time four wheel conversion. Mile Marker says to use 10-40, the owners manual says to use 10-40, other falks I have talked to say to use ATF. When I dumped the fluid to do the conversion it was full of ATF. I was told to run ATF because if the seals go out between the trany and transfer the engine oil would wipe out the trany. I was also going to run full synthetic, but I was told the RTV I used to put the transfer case back together with is be eaten by the synthetic fluid. Does anybody have any input on this? Thanks, Dana __________________ Dana 77 K-20 454 73 SS El Camino 454 68 SS Camaro 327 48 Chevy 216 755 John Deere

12-19-2003, 11:13 PM	#2
INTIMIBLAZER Registered User Join Date: Sep 2003 Location: PHILA. PA Posts: 138	the manual does say 10w40 but i would use dexronIIIjust because of the oil mixing problem as far as synthetic is concerned i have no first hand experience hope this helps,Keith __________________ 85 K5 BLAZER 305 TH400/208 3.08 REARS NO LIFT 95 DODGE RAM 2500 MAGNUM 360 4X2 EXTEND CAB-LONG BED If you can read this, thank a teacher. If you can read this in English, thank a veteran.

12-20-2003, 03:46 PM	#4
Scrub Collector of rusty Items Join Date: Dec 2001 Location: Sisters,Oregon USA Posts: 731	You might get ATF to go from the tranny into the case, but I doubt anything will go from the case to the tranny. My 205 was howling a bit and I pulled the fill plug to check the level. It had at least 2 quarts more ATF than you could get into it through the filler hole. I drained it and refilled with 70-125 Gear lube. The howling stopped. I may have done a bad there, but it is still going just fine 3000 miles later. No gear oil is in the tranny. verdict: The ATF was comming from the tranny. __________________ Scrub Sisters, Oregon - Home of the Sisters Rodeo. 70 GMC 1,000,000 + miles 72 K-20 project, 456 Dana60 front, Corp14 rear w/locker, 265R19.5 tires 20-ply. Warn 12k winches both ends, Cross-over steering with raised tie-rod, Powerbox steering, 4500 watt 120-AC power, Air, Hydraulic aux power, 4 inch lift, 5000 lb air-bags both ends.

12-29-2003, 03:15 AM	#6
Cethern What was I thinking?? Join Date: Dec 2003 Location: Commerce City Co Posts: 636	Use the Synthetic No matter what oil you use I would go with the Synthetic. It will not eat anything up. My Blazers and other cars I've had have used Synthetic oil for everything from the Tranny to power stearing and the diff's with no problems. As for what Oil to put in the 203 go with what the book says. I found it weird to add ATF to my old 208. And I think there are two seals so if your vents are some what clean you should be ok. Cethern

12-20-2003, 01:07 AM	#3
kwmech Senior Member Join Date: Mar 2003 Location: Colfax-California Posts: 8,684	Over all the years I have been working on 4xs I've seen atf, 10-40, 30 wt. and 90wt in these boxes. Doesn't seem to matter what is in them as long as there is some lubrication.

12-27-2003, 01:53 AM	#5
rogf4 Registered User Join Date: Nov 2003 Location: whitesburg,Tn Posts: 223	I would run the 10/40 or syntec. the trans will over pressure the transfer case but not vise versa The heavier oil seems to run quiter and give better lubrication



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