What the log is for
A service log is a running list of the work done on a vehicle: oil changes, tire rotations, brake work, belts, fluids, and any repair that touched a safety system. Each entry should say what was done, the odometer reading at the time, and the shop or person who did the work. Its purpose is simple. It lets a later reader, whether you, a service advisor, a buyer, or an insurer's adjuster, see whether the maintenance in the owner's manual was followed.
Start with the owner's manual
The maintenance schedule in the owner's manual is the list to check the log against. Read the interval table. Most schedules trigger work by mileage, by elapsed time, or by whichever comes first, and many include a severe-use section with shorter intervals. Find out which section fits your driving. Then compare the log to the table line by line. Gaps are easy to see once the list is written out side by side.
What each entry should contain
Ask for an invoice that lists the work, not only a total. Look for the odometer reading, the day the work was done, the vehicle identification number, and the name and address of the shop. Ask whether parts are listed by part number. Keep the paper or the digital file together. A line that reads service performed is weaker than a line that names the fluid, the filter, and the inspection that was done.
Questions to ask before the work starts
Ask whether the work will be recorded against the manufacturer's schedule, and whether the shop will give you a copy of the record for your file. Ask which parts are new, which are rebuilt, and which are aftermarket, and whether the replacement is the part the manufacturer specifies. Ask whether any work is being done that is not in the schedule, and why. If a shop cannot say what it did, the log will not say it either.
Work you did yourself
If you change the oil or the filters at home, keep the same kind of record. Note the day, the odometer reading, the product and its grade, the filter part number, and where you bought the parts. Keep the receipts with the note. A self-done entry is still an entry, but it is only as useful as the paper behind it.
Warranty and service contract paperwork
Some maintenance requirements in a factory warranty or a separate service contract depend on proof that the work was done on time. Read the maintenance section of the written terms before you assume what the log must show. This page does not describe what any particular contract requires. It only points you to where the answer lives: the terms in your own paperwork, and the factory terms for your make on this site. Keep the log beside those documents.
When the vehicle changes hands
A complete log travels with the vehicle better than any summary. A new owner, a dealer, or a service advisor will read the entries and the odometer trail to understand what was done and what is still due. Put the owner's manual, the maintenance schedule pages, and the log in one folder. Note any item the schedule shows as due but not yet done, so the next owner does not have to guess.
What to check at the next visit
Before each visit, compare the log to the schedule and mark what is due. Ask the shop to record the work against the same line of the schedule. If the log has a gap, ask whether the work was done and never recorded, or whether it was skipped. Write the answer in your own words beside the entry. The record is only as good as the questions you ask and the paper you keep.