Sounds like you have a bigger problem than what oil to use. Time to start documenting. Pick your battles and make sure you are being reasonable.
Make your requirements clear - be specific, ideally in writing. [Can the employee read? Illiteracy is said to be very common.] If/When your requirements are not met, meet with the employee and note the problems. After a few of these meetings, provide a verbal warning that things have to change or there will be consequences (keep a record of the date and content), then a written warning including a reminder of the verbal warning and the caution that there will be further consequences unless things improve - up to and including loss of job, then (possibly) days off without pay, and finally termination.
An important rule of thumb is that an employee should never be surprised that they're being fired. Most people will be horrified and smarten up with a verbal warning. If they don't understand that a written warning is serious (and I've seen it) they're pretty dense.
Assuming the mechanic has enough time to do the work, you should be able to reasonably expect record keeping for all vehicles under his care, oil and filters changed at the manufacturers/oil monitor's recommended interval, air filters changed at reasonable intervals, periodic safety checks, etc. I would either provide him the oil and other parts or specify his budget ($2.50/quart or whatever is reasonable at that location).
Give positive feedback too. Employee management is not just pointing out negatives. If he's doing a good job in some area say so.
If you do your job things will improve. Either the work will become satisfactory (probably never great, but at least satisfactory), or he'll quit or be fired.
In my experience other employees will be annoyed that "nothing is ever done about XXX" but when you do something about it, they'll be shocked. But over time employees will notice that the problem areas are getting cleaned up and morale will actually improve. Been there, done that.