Straight 40 question

Not open for further replies.
Jun 20, 2003
I live in GA and have a '90 Jeep Wrangler (160,000) with a rebuilt engine with 18,000 miles on it. I recently had a tune-up and some gaskets replaced because of oil leaks. Also had a new radiator installed. The mechanic put Castrol straight 40 oil in the car and told me that I should use this oil all the time. Could someone give me some feedback? Thanks
Well he apparently doesn't want this oil to leak out the gaskets. He's probably not sure of his work.

32 F is as low as you would ever want to go and at that temp its like starting at about -5 F with a 10W oil. So really I'd want to stay conservative and not use this oil below about 50F.

From a practical standpoint there is no reason to consider a straight 40 wt. if his gaskets leak-they will leak at operating temperature in which case a 10W-40 and a staight 40 wt are the same. I'd like to know his reasoning - but it's probably not worth the time listening.

If the oil is labeled that it meets API Service Category SJ (and Castrol Heavy Duty does meet SJ), it's OK, but wouldn't be my choice. If the straight 40 is Service Category SA or SB, it's not suitable for any modern engine.

With 18,000 miles on that engine, I'd run 10W-30. If the oil pressure was low because the rebuilder didn't do a very good job, I'd use 15W-40.

No engine that's well taken care of should need a rebuild at 140,000...what happened to yours?

Thanks for the replies. This is my 21 year old's jeep and he doesn't take good care of it at all. Since neither one of us knows much about engines - I told him to have it changed every 3,000 miles and watch the fluids. He apparently didn't. This guy has it sounding like it did when it was new. It runs smooth and idles great - which has always been a problem - even when we bought it new. I became concerned when the guy at the auto shop ask me why I was buying straight 40. He thought it was a bad idea also. Would be ok to run straight 30 when it gets cooler?

If the oil pressure was low because the rebuilder didn't do a very good job, I'd use 15W-40.

The 4.0 (and the other versions of the AMC/DC I6) have had various oil pressure "standards" over the years. If you look at the Chiltons Manual it will say something like 13-39 at idle (or something like that). My brand new 99 4.0 was around 13 at hot idle (I went to a high volume oil pump) ..but the very next year they had about 24 at idle (I have 42).

I would not use straight 40 weight unless there was some other operational anomoly that it could cure (which would only stretch the service of the engine until it is totally toast).
Straight SAE 40 is great for like stationary power in continuous operation, few startups, marine apps, although marine is going multigrade. In a vehicle it doesn't make alot of sense because of the way it's used.

It works well when the engine is operated at a constant speed for very long periods of time, the oil is at the proper temperature and becasue the base oil is close to the actual viscosity of the oil it lasts for a long time and is cheap. Multigrades can cause ring fouling in this particular instance because the additives used to create a multigrade can breakdown on the hot piston rings and over time build up causing fouling. This may not be true anymore.

The only time I ever heard of anyone using straight weight oils is a friend of mine who had his oil changed while on a trip to Mexico (SAE 40).

In your application the oil will spend alot of time below operating temperature and will be exposed to hundreds of startups. Under these conditions a multigrade oil is much better. A "thinner" oil such as 5W30, 5W40 will quickly arrive to the bearings and valvetrain on startup. This is very important to the longevity of the machine, greater than 50% of wear occurs during this period for the life of the vehicle! Also when the oil is cold the oil pump will bypass a certain volume of oil to keep the lube system within safe pressures. With a straight 40 weight oil enough volume may bypass that the volume of oil is insufficient to protect the motor if under load for the fist few minutes driving even though the pressure is correct!

I find it hard to believe that 'thin' 10W40 will be almost exactly the same viscosity as your SAE 40 at operating temp, but it's true. Multigrade engine oil has allowed engine manufacturers to build much more precise machines for the everyday consumer. Without it you couldn't build the highspeed, highpower automobile engines of today.

Hope this provides some rationale to the use of "thinner" multigrades. If the engine was rebuilt properly any premium quality multigrade engine oil should do the trick. These are popular grades for gasoline engines: 5w30, 10w30, 10w40. The actual grade should be called out in the owners manual, it is the correct grade so you can't really go wrong with it.

BTW save that bottle of Castrol SAE 40 'till it's winter, leave it outside overnight and give it a shake. Let me know what you think.
Thanks - so on the next oil change will it be ok to just drain the straight 40 out and put in the 10W40 or 15W40 or should I run some Auto RX also?

Originally posted by Pablo:
15W-40 is the oil you want.

I agree. Rebuild clearances are rarely as tight as factory new, so 15w40 should serve you well year round in your climate. Chevron, Mobil, Pennzoil, or Shell would all be good choices.

I would go ahead and change it now. If the rebuilder did his job correctly and all internal parts were vatted, you shouldn't need an AutoRX treatment.
The only time I used straight 40 was in a 175,000 mile Toyota with bad valve seals that smoked a lot. In theory, 20W-50 would be better. In real life, it burned less oil on straight 40 than on 20W-50. I think that is because oil temperatures in that engine were lower than the 100C where the SAE viscosity is determined. This would apply to a leaker as well.

Castrol states right on the bottle that SAE 40 is recommended only for use above 60F, 40F for SAE 30.
Not open for further replies.