Here's my personal opinion. I will NOT guarantee that it's the best
approach for every situation, but it's what I'd do:
I'd suggest using a multi-grade synthetic oil. Years ago, multi-grade
oils had relatively poor high-temperature shear strength
characteristics, but this problem has been largely overcome. Today's
non-synthetic multi-grade oils are much improved in this respect, and
synthetic oils are even better.
Synthetic oils can provide a wider viscosity index range while
maintaining excellent lubrication characteristics over a wide
temperature range. There are several ASTM standardized tests designed
to measure lubrication characteristics. Synthetic oils outperform the
non-synthetic oils. They also flow better at low temperatures and bond
more firmly with metal surfaces, leaving a longer-lasting film that
reduces dry-start damage. They don't "coke" as easily on hot surfaces.
They will improve (slightly) fuel economy and power output by reducing
internal friction in your engine. Hot idle oil pressure should go up.
I've switched from non-synthetic to synthetic oil on every new and used
vehicle that I've owned over the past 15 years, and I've always had
good results. That includes six years in Minnesota, two of which were
in Brrrrrmidji, with 40-below zero F mornings quite common.
There are some legitimate additives that can impart at least some of
these superior characteristics to non-synthetic oils, but considering
the costs vs. benefits, I think I'd still opt for the synthetics.
I won't suggest any particular brand. I'd try to find actual test
results and do some comparison shopping. You probably won't find all
brands in any one published test, so you may have to gather up several
different sources. There are a bunch of oil-related sites on the
Internet that contain useful information, but don't take their opinions
at face value. Many of these sites have vested interests in particular
brands and may "conveniently" forget to mention some negative aspect of
their product or the test results of a competing product that happened
to score better in the tests.
Another thing to consider is how to safely introduce a detergent oil
into a dirty engine. If a non-detergent oil was routinely used in the
engine, there will be deposits of sludge, varnish, etc., that collect
in many spots. If an engine has been run for a long time without a
thermostat (usually, but not always, running too cool), the problem can
be MUCH worse, with a thick coating of sludge covering much of the
interior surfaces of the block. When you switch to a detergent oil
(which virtually all popular types are), these deposits tend to loosen
up and fall or flow into the oil. However, this is normally not a
problem unless the deposits are so extensive that they manage to clog
up the oil pickup screen before they can be trapped in the oil filter.
In any case, you'd want to change the oil and filter two or three times
over a short period of time if you are introducing detergent oil into a
dirty engine, and it might be a good idea to remove the oil pan after
running the detergent oil in the engine for a little while (hard to say
exactly how long) and clean the pickup screen, just in case some larger
deposits came off in chunks and began clogging the screen.
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