engine cocktail or recipe for DISASTER??

[daddiojiggy]

let's get past the question of why before we start.i've got 4 litres(each) of 5w30 and 20w40 and i need 7.5 litres for my engine.now i know some of you have jumped ahead but i'll ask the question anyway.....what would happen if i combined the two THOROUGHLY and used the concoction? as an aside i'd like to take an informal poll on oil changes throughout the membership for those that did or still take their cars to dealers for oil changes.i'd like responses to go as follows:
'98 c230 $66.00 aug'01 texas--- where we tell car,cost,date and location .thanks,in advance

[Mike Richards]

Daddio:

Not meaning to be a smart-ass, but by my calculations, you'd end up with a home-brew batch of 12.5W/35.

Are these 2 different weights of the same type..ie..conventional or synthetic and from the same mfg? I could not say if that would really matter.

Hopefully we'll soon here from someone here with a high-end degree in chemistry and a background in the mfg/analysis of automotive motor oil.

Can't say about dealer oil changes. Have never paid anyone to do that.

[daddiojiggy]

no offence taken mike.however something didn't seem right so i checked and the oils are 5w30 and 20w50 and yes they are the same type.i thought i'd check with the brotherhood BEFORE i did it-that is if i was to do it

[engatwork]

Pour it in and go. With it being summer (95 dF around here during the middle part of the day) put all of the 20W50 in and then top it off with the other. You will never know the difference.

It's been many years since I studied that subject, but I believe that if you mix equal parts 30 and 50 weight, you get something like 37, not 40. On the other hand, the system for designating the viscosity also has rather broad borders, so some of the cheaper stuff being marketed as 40 is probably 37 and the next cheap bottle might be 43.

To put it another way (the way I first encountered the issue), if you have a case of 50 weight and a case of 30 weight, and you want to put 40 weight in your engine, you should use two 50 weight to each 30 weight.

Complicating the matter further, if you try to mix different brands, is the problem that some viscosity improvers complement one another better than others, so you can't always expect the high temperature number to come out in direct proportion to the low temperature numbers when you mix different viscosities of different brands of multigrades. Really, though, these ranges are flexible enough that it shouldn't create a problem in any case.

[daddiojiggy]

thanks f.f.,i had a feeling that it was more than a simple matter of adding/dividing but i guess the main thing is i;ll be safe,by the by,f.f.,what doesthat mean at the bottom of your signature?

"Quothe Obi wan Kenobi: "May the Force be with you" (using the more personal theological interpretation of "The Force" -- crude materialists may prefer to substitute _vis_ for _numen_, but in either case, it is clear to me as a Latin professor that the verb should be _adsit_ with the dative of interest _tibi_, not "sit tecum," as many Latin students would put forth as their first guess).

Sometimes I also use "Dixit Arrius Sordidus: 'Perge, ut dies mihi perficiatur!'" ("Said Dirty Harry: 'Proceed, in order that the day be completed for me!'"). Both of these are very useful for illustrating certain fundamental differences between Latin and English style and structure in class. My nom du keyboard, _Fimum Fit_, however, would best be translated as "Dung Develops" (Imitating the alliteration of the Latin) or "Excrement Occurs." More colloquial translations are, of course, also possible.