Recently when I changed my engine oil for my 7 year old Honda Oddesey, I was asked to choose between 20w50 or 5w30 of the same grade.I was using 20w50 and I decided to stick to that.any advice pls before my next oil change.Thanks

Limited to those two options and having the history of using the 20w50 I would have done as you did. Now you could also ask them to put 2 parts 5w30 with 1 part 20w50 to roughly approximate a thick 10w30 or thinner 10w40. However, that is not ideal. Where do you get the oil changed? Why don't they stock any other grades? What does your book call for? What is the lowest temperature you will have to start the cold engine in? Not knowing those answers, and assuming 20w50 was working fine for you. I might suggest something a bit thinner unless prohibited by the book. Either a 15w40 or 10w40 would be a much better choice, but can't be sure with out more info.

TallPaul,
I have the oil changed at a workshop.Our tropical climate here is hot thru out the year.
I was wondering why such viscous oil is marketed here.

I only use 40/50/60 weights in UK and its not warm.

Although sump temperature goes up by 0.3c for 1c deg ambient rise. Is there much difference in the oil temp actually in engine. The water is still cooling engine to a stable temp no matter what ambient temperature.

Also how quickly do minerals shear out of grade
20W50 mineral compared to 5W30 Synthetic?

Sandman: Given that it is a tropical climate, hot all year, I really wonder why they carry 5w30 (I would not run it in your climate). A 20w50 probably performs quite well in your climate, however, a 15w40 may work well also and is a good bit less viscous (50 wt about 19 cSt vs 40 wt about 14 cSt). If you have an oil pressure gauge and the pressure is not running on the low side, you could try 15w40 next time.

MGBV8: If a 20w50 sheers I can't imagine it getting much thinner than a 30 weight, but a 5w30 can sheer to a 20 weight or maybe even less. I have heard that a 20w50 is more or less a straight 30 with viscosity index improver added to get the 50 rating and that the straight 30 already passes the 20w test (or with a little help from some pour point depressant).

TallPaul

If Amsoil synthetic then may even thicken!

Amsoil synthetic 20w50 I would think has no viscosity index improvers, no? So would it even sheer?

What does thicken oil, besides coolant.

With high temperatures and hard use, the base stock gets thicker while the V.I. improver shears down. What you are left with is a thick oil with a very low viscosity index.

Soot and fuel dilution work in opposite directions on vis.

Amsoil may burn off but not shear!

In high temperature race situations its not necessary to increase vis with an ester based oil unless fuel dilution becomes an issue. In the old days the way to improve HTHS was to increase vis but with improvers that then shear.

With classic cars the standard oil is mineral 20W50 but this is close to edge in winter at say -10c which is why Castrol now recommend 15W40 mineral, better cold starting and less shear the minerals 30 years ago.

Anyway, back to question, the US has a wide range of temperatures but current oil recommendations are xW20 or xW30, although this may be a mpg v wear issue, with better oils and fuels the lower weight oils seam to work.

My preference is for a xW40 or above but exactly what grade and Honda recommendations would be useful.

Oxidation thickens...sometimes insolubles...

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Originally posted by TallPaul:

What does thicken oil, besides coolant.

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Both severe high and low temp operation and blowby gasses affect certain additives/ oil formulas along with long periods of exposure to air .

If I lived on the equator I'd still trust a real 30wt synthetic and it's film strength vs a 20w-50 mineral oil .... especially @ wot with 7k miles on the oil assuming no fuel was present in the oil . I've seen domestic V8 engines shear 20w-50 mineral oil to a 30wt in 3k miles .

quote:

I've seen domestic V8 engines shear 20w-50 mineral oil to a 30wt in 3k miles .

Sure that you saw shearing and not simply fuel dilution?

sandman - if you're specced for a 5w/30 I'd recommend a 10w40 for tropical conditions. Im from northern Australia and know what a difference it makes. I do know that a 20w/50 may be too heavy and cold starts can leave your engine sluggish - four cylinders often feel like they're driving with the handbrake on until the engine is significantly warm. If you need to go to a 50 - use a 15/50. Else 10 or 15/40 is great.

quote:

Originally posted by Callisa:

quote:

I've seen domestic V8 engines shear 20w-50 mineral oil to a 30wt in 3k miles .

Sure that you saw shearing and not simply fuel dilution?

Quite certain

quote:

Originally posted by Timer:

quote:

Originally posted by Callisa:

quote:

I've seen domestic V8 engines shear 20w-50 mineral oil to a 30wt in 3k miles .

Sure that you saw shearing and not simply fuel dilution?

Quite certain

Well, there are two ways of producing a 20W-50.
Use a 10W-30 mineral base oil and use some viscoity modiiers, or simply use "heavy" mineral base oil with few VM.

How much oil does the V8 have?