You could try mixing equal parts 10w-30 and 20w-50, but I don't know if you will get exactly a 15w-40 grade. It would be easier to run the 20w-50 in hot weather and run the 5w-30 or 10w-30 in cooler weather.
Just mix whatever you want. To state mix this wt with this wt will produce this viscosty without any kind of testing is nothing more that, guessing. To my knowledge there is no formula that will give you that either. Put in a thicker oil, then put in a thinner one to thin out the thicker. Or put in a thin oil, then add a thicker one. What will you end up with? who knows. Your guess is as good as anyone else on here.
If you truly want a 15w40 oil, the best thing to do is buy that grade to begin with. Mixing it
yourself isn't harmful, it's just that you may never truly know what viscosity you're ending up
with in the end. Also, lately I have noticed that even within the same brand, there is a vastly
different additive package with the thicker weight oils compared to the lower ones, so there
could still be an additive clash there.
I mix 10w/30 and 20w/50 myself because I have plenty of both on hand and in my 50k+ vehicles I use thicker than 10w/30 oil. I don't use(prefer) 10w/40 due to extra high amounts of VII, but 10w/30 and 20w/50 don't have to use too much of it. The oil mfgrs that I have called have all told me that there is no problem with mixing as far as any conflict of additives, but that they do make a wt that I can use to save the trouble of mixing myself. I would use 15w/40 if I didn't have plenty of the others already in the garage for other vehicles. As for trying to make an exact wt by mixing, probably should not get too hot/bothered about that, even 10w/30 oils have a range of actual viscosities among brands that still qualify as 10w/30, other wts do the same thing, I mainly just want to use thicker than 10w/30, and thinner than 20w/50 in certain circumstances. A 50/50 mix may not be exactly a 15w/40, but it is close enough for what I want.
[ September 02, 2002, 08:08 AM: Message edited by: ZR2RANDO ]
two things Rondo, Not to be sounding rude, just wondering,
1- have you ever done an oil analysis on your mixed oils?, If so, would you share your formula of that mix, miles, engine, ect, and the #'s on that analysis so we might see just how good this mix is in comparision to what you'd get from the chemists mixing the oil. I think it would be interesting to compare your's against theirs.
2- If what you are wanting to do is increase an oils protection ability, then look at stp(thats right, you do use stp also don't you, forgot) or better yet Schaeffers 132 EP additive. These are designed to increase the viscosity of any oil by adding a heavier dose of EP additive and VI's, therefore more hydrodynamic properties, more clinging, and more barrier additive.
Back to you Ruberto,
I have NOT done any oil analysis, it is not worth the money to me. I change oil every 3k miles and pay about $7 to do it. I am not trying to be an oil change scientist here but I am just doing what has been working for me for about the last 25 years. I know oil chemistries have changed over the years, but the basics of lubrication have not.
I am using STP with 10w/40 at the moment in one of my trucks, it works fine as long as you don't use it for extended drain intervals, that truck has 185k miles on it at the moment (1989 Nissan 4cyl 4x4). I am just going to start using 20w/50 instead so that I can decrease some of the VII for a while .
I have no interest in using Amsoil/Schaefers/Royal Purple etc type oils, to me there is no benefit in paying that much money for those products when my history with vehicles has shown that my methods work well for me. Over the years I have had one premature engine death and that was the only engine I ever stayed with 10w/30 oil (not blaming the oil just stating the fact), it had bearing problems at 107k miles.
Normally I consider at least 200k miles as the time I start thinking about getting a new vehicle.