The whole topic on oil viscosity is confusing to me. This whole summer I have been using a thicker oil Motul Synergie 15W50. I was using Castrol 10W30 but it seemed to just burn up. I have read through my owners manual and it looks like 10W30 is fine for Arizona. Some people I know use 5W20, 10W30, 0W50 even and so on. It's going to to be getting colder here in the next few months and was wondering if it is safe to keep using 15W50 Motul. Should I back down to 10W40 possibly? My motor in my 86 300zx has 140k miles on it.

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1986 Nissan 300zx
5-speed
Digital Dash
Almost all stock

Also, I have been using Mobil 1 75W140 gear oil in both my manual transmission and differential. Should I be using 75W90 in both or just one or the other?

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1986 Nissan 300zx
5-speed
Digital Dash
Almost all stock

Well, I'm not sure about the gear oil questions you have, but I've run different weights in my Z before and never really noticed any issues. I've run 10w30, 10w40, and 20w50 and have never noticed any issues regardless of the time of year. The only reason I usually run a heavier weight is to help prevent the lifters from ticking which sometimes happens when out on the track or during cold starts. I personally don't see any reason to switch your oil unless the temps get so low that it is outside of the recommended operating range of the weight you're using.

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Mike
TZC Treasurer

1988 300ZX 2+2 Turbo
1972 240Z

Hi there I use 10w40 the biggest problem with differant oils is start up is hard on the engine and if oil dosn't get to engine and the turbo fast enough it will shorten the engine life and each car is differant.

1. is your car burning oil.
2. is it smoking
3. why did you change your oil from 10w30
4. how hard are you driving your car
5. how cold dose it get in the winter?
6. how hot dose it get in the summer?
7. how much oil pressure do you have when it's at idle at temp.
8 are you running turbo's?
9. how much boost are you running?
I'm just asking? I don't change what is recomended in less there is a problem (ie there is not enough oil pressure ) I hope this helps if not sorry!!!!!

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1998 nissan stagea 260rs rb26dett
1957 nomad pro charger
2000 bmw 528it

Here are the answers to your questions.

1.) No I am not burning any oil.
2.) Not smoking at all
3.) I was told by a tech at work that 20W50 will provide better protection in the summer for an older engine.
4.) I drive my car normal. Never jump on it.
5.) Well it goes down to the 20s to 30s at night here in AZ like ya'll know.
6.) 20W50 worked good in the summer. Felt like the motor was lagging though versus 10W30.
7.) 30 - 45 psi at idle
8.) N/A
9.) N/A

I figured I would switch to the 10W40 Motul for the winter and colder temperatures here.
Do you have any input on the gear oil weight?

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1986 Nissan 300zx
5-speed
Digital Dash
Almost all stock

I'm sorry pm me next time I have been busy jengols@yahoo.com

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1998 nissan stagea 260rs rb26dett
1957 nomad pro charger
2000 bmw 528it

Here's what the deal is. I've asked this same question to techs and people on Z forums. Be warned that this is my opinion as I'm not a "certified" mechanic and that everyone and their grandmother has an opinion on motor oils. Firstly as for gear oil I used Red Line MT-90 in the transmission and 75w90 in the rear end. The car seems to be happy. Using 140 weight (such as 75w140) is good for the summer but when it starts getting colder (like it is now) the oil tends to get really thick and makes hard to drive. The gears get a bit sticky and loud when cold. In the summer when the low is like 80 by all means using it isn't a problem at all. But it just a pain to switch between the two when seasons change.

As for oil. I've always used 10w30 (or 10w40 in the summer ) in all the cars I've owned. I have a 1996 Jeep Grand Cherokee that my brother now drives that I've owned since new and it currently has 260K with no engine issues at all. As for the Z, I've only owned it since August so it only faced the summer weather for about three weeks. In that time I never really noticed that using light weight oil was a problem. It didn't make any noise really and the pressure was pretty steady at all times. In reality mate its all how you feel. For me 20w weight oil of any kind is just too thick at start up for my liking. I personally use Valvoline VR1 (10w30) oil at the moment as it has a high Zinc content then most oils. I would think that 15W50 is a little heavy for even a Z engine. There are plenty of articles on other forums about oils.

Here are the big things remember:
1. Stick with whatever brand you go with. Don't flip and flop and switch between brands.
2. Use oils that have high ZDDP (zinc) ratings as older cars such as Z's need it.

Hope that all helps
Jan

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1976 Datsun 280z 5 speed.